To me that looks like decorative paneling. The lego bits are shaped in imitation of the pegs that were neccesary to secure wooden structures and it is a common element of Greek style temple architecture that eventually was diffused out to other building types.

Granted, it is a little bit tricky to tell without having a wider view.

EDIT: OK, to be slightly more expansive, for one, I need to reiterate that I am not positive (I am pretty sure though), but this is what it looks like based off of my experience with Greco-Roman architecture. That particular block would have originally been part of the roof--you can see a bunch of examples on this page. The technical term for this is a "skeuomorph"--something made out of one material to look like another. A classic example is pottery that imitates the forms of bronze cauldrons, or from the modern world, the "wood" paneling on the side of some cars. The idea is that Greek temples were originally made out of wood, which required wooden pegs to fasten the various elements together. When they started building in stone, they carved out those bumps to imitate the appearance of wooden pegs. Why? Because they wanted too.

My assumption would be that you took that image near a temple or other large public building, but honestly the Romans were pretty free with their stylistic borrowings in architecture.

This is a "mutulus" decorated with 3 rows of 6 "guttae". Its part of the "Geison" in the doric order. The most famous example of a temple of the doric order would be the Parthenon in Athens where you still can see the mutuli in place. Modern iterations of the Doric order like the Brandenburg Gate has them too.

Yiu can see them in place here for example. The Geison is the horizontal beam below the pediment. The mutuli are located on the underside of the Geison.

This particular part is probably part of the HErcules and Minerva Temple which was a doric temple in Pompeji...but i would need to look that up.

Source: Studied Archaeology specialized in greek architecture...but sadly never got to finish it.

€dit: To expand a little:

If you look at this photo of the Parthenon. The plain area directly above the collums ist the architrave. On the upper edge of the architrave you see another example of guttae, this is called a regulus. The slim bar between the architrave and reguli an the picture fields above is the taenia.

Above the taenia there is the triglyph architrave. The triglyphs are the parts directly above the reguli. There is one above every collumn and above the center every Intercolumniation (the space between two collumns) except for the edges, but the doric corner cflict is another topic. Between the triglyph are the metopes, the part where the friezes are.

and finally above every metope and triglyph there is a mutulus. And these mutuli are decorated with 3 rows of 6 guttae. The beam where they are attached to is the geison. The little edge above the Geison is the sima. ABove that, which is destroyed here is the Tympanum, which was decorated with friezes or statues. Above the tympanum the raking geison with the raking sima follows.

€dit 2: Further information and source:

As u/tiako correctly said. Greek stone architecture is derived from earlier wooden buildings. They've changed the the material but wanted to retain the look. This developed further and later temples of the ionian order lost more and more features that where derived from the earliest wooden temples. MOst of the parts i mentioned served decorative purposes only. The structural engineering of a stone building and a wooden one are vastly different. The triglyphs for example are probably stylistic element whoch are meant to resemble the bearing beams of the wooden temples, which weren't nbeeded anymore (not in the way they were in the wooden temple). If you look through greel temples over the centuries you'll see that they they all somewhat similar across the two order (doric and ionian) as if they were build from a guidebook. Only few temples left that "stencil" like, again, the parthenon for example which serves as a bridge between the two orders. Is basic appearance is doric, but it got ionic elements. But i digress. Those parts are all decorative and would not be needed for the building to be standing. Some parts like the sima served as kind of a rain gutter and there where even waterspouts on them. The mutulus and guttae, to be specific is what we archaeologists in germany call a "typologisches rudiment" or typological vestige. It once served a purpose but its now just decoration because thats the way it was always done. The Mutulus and guttae is for eaxample one thing thats missing on ionian temples.

Sources:

Sadly i only have german books in my bookshelf right no, i don't know how much good would that do. this is a scan of a diagram from "Klassische Archäologie Grundwissen" by Tonio Hölscher which is a very good introduction on classical archaeology. A good english source might be the "Handbook of greek and roman Architecture" by D. S. Robertson. Its from 1943 but the basic principles and names haven't changed much since then. Another good read on the architecture of the Acropolis of Athens would be the publications by Manolis Korres, buts thats all german too.

Do you have an image of the larger area to give a sense of scale and context? That will likely make it much easier to identify them.