When it comes to keeping algae in check, the Margarita Snail - Margarites pupillius - is a great choice. The shell is dark grey or black, and rounded over. They do quite well in reef environments, and even when flipped over can right themselves and continue on their way. Reasonably priced, adding these is a great plan. Hermit crabs aren't interested in their shells, so they are safe from predation. Reef-safe.