It indeed seems to refer to Antoninus Pius, since the description talks about Imperator Caesar T. Aelius Augustus Pius. The T probably means Titus, and his full throne was Caesar Titus Aelius Hadrianus Antoninus Augustus Pius.

After that, there follows a reign year: the year of his third consulship (COS III) and it seems to say his second year with tribunicia potestas (TRIB POT II)? The rest talks about him renewing and finishing an aquadcut, possibly one his predecessor has started or something (AQUAE DUCTUM INNOV(AVIT) CONSUMMAVIT)

More likely than not it refers to Hadrian, since he was a philhellene and heavily patronized Athens. He even finished the Temple of Zeus that's near where you found that inscription. There's also the fact that it says his name after his titles: "Imperator Caesar Aelius" (His full name was Publius Aelius Hadrianus Augustus).

Then again, Antoninus Pius's was also named Aelius (after his adopted father Hadrian!) so it could possibly refer to him. I don't know of any connection he had to Athens though. Someone who can read the Latin could possibly give you more information. That part at the end looks like it might be a date, or reign year, or something.