I'm not familiar with any coins of Abd al-Malik showing a cross, if you could point me to what you're looking at I would appreciate it.

That being said, coinage under Abd al-Malik is a fascinating topic. The earliest post-conquest coinage was simply re-castings of Byzantine and Sassanian coinage. So, for example, early Arab-Sassanian silver dirhams were identical to the previous coinage, but with "bismallah" stamped around the outer edge. In formerly-Byzantine Syria, the new Arab rulers continued to mint coins from Byzantine molds.

They did, however, almost immediately begin removing the "Cross on Steps" from the reverse of the coinage, even as the emperor remained on the obverse (as did fire alters in the Sassanian-minted coins). So you end up with either "Column with bar on steps" or "Column with globe on steps" coins that look like this: http://www.muslimheritage.com/sites/default/files/umayyad_coins_02.jpg

Abd al-Malik was the first ruler of the Arab-Islamic empire to institute more serious coinage reforms. His "Standing Caliph Dinars" show a bearded figure (presumably Abd al-Malik himself, although there is reason to believe that it might be intended as a depiction of Muhammad) with the column with globe on steps on the reverse and Islamic religious inscriptions written around each side: http://www.coinweek.com/wp-content/uploads/2014/12/standing_califh.jpg

The coins retained the column on steps in place of a cross potentially as a means of signifying the equivalency in value of the coin, though what the object itself might have represented is obscure.

More radical still was the change in coinage introduced only a few years later when Abd al-Malik removed the human image from the coins entirely and introduced purely epigraphic dinars, that is, coins with only engraved text. Typically the shahada or verses from the Quran.

These coinage reforms coincided with significant changes in the role of the state and the status of Islam. The relationship, for example, between Muslims and other people of the book (Jews and Christians) appears to have been quite a bit fuzzier before Abd al-Malik. The Dome of the Rock is perhaps the most notable example, the inscriptions on which consist largely of polemical verses from the Quran directed at Christians. He is likewise credited with Arabizing government administration more generally, where previously bureaucrats often conducted business in bilingual Greek and Arabic.