The Fallen Monk Pirates seem to share the same dark skin tone as their captain, Urouge, and even wear the same brown monk robes. It is unknown whether Urouge is the only one to have come from a Sky Island or not though it seems likely as he is the only one so far shown with their trademark wings.
It can be assumed that they are strong because of their captain's high bounty and the fact that they made it all the way to Sabaody Archipelago. Urouge is also noted as one of the Eleven Supernovas on Sabaody Archipelago. Despite Urouge being knocked out, the crew was still able to escape from Kizaru's attack.