Saltwater Hermit Crabs not doing Their job!?

I'm getting a little bit of green hair algae growth and I have 2 hermit crabs (a hawaiian reef hermit and a blue legged hermit) but don't seem to be doing anything about the algae problem. Do I need more hermits maybe? It's a 33 gallon tank. Does 2 one inch hermits just not quite get the job done well enough? How many hermts should I add to take care of the algae before it gets too bad? I was thinking about getting more blue legged because I love the color of them!