I have all the common, rare, and legendary skins for the croc and it didn't give me the achievement..

Mystic skins

Are you sure it didn’t give you the achievement? Go to your game menu and click the trophy, then scroll near to the bottom where it says something along the lines of “Unlock All Crocodile Skins”. If you hadn’t collected it yet, the star will be glowing. If you have collected it already, it will be dimly lit. If you haven’t completed it at all, it will be dark.

If the star is dark, make sure you’ve actually unlocked all the skins of the crocodile, and that it’s not another animal you might’ve looked at by mistake.

If all else fails, contact TRG’s customer support via the in-game menu.

I hope I could help.

I think it means that you must have the eyes clothes and accessories for it.