Percnon gibbesi, Sally lightfoot crab. Through a scientific study made on the wild populations in 2006 by Puccio et al., the species is now considered a strict herbivore. As such this crab would keep your algae under control. Always see plenty of these crabs at Pasir Ris farm, Ah Beng's place.
http://www.ingentaconnect.com/content/klu/...f41pb5.victoria