I agree, get rid of the killer crab. I love mexican turbo snails and margharita snails. Smaller hermit crabs like blue leg hermits are a great addition also. Shorten the lighting period or even go a couple of days without lights.
Ultimately though, good maintenance is the key. If you keep up on weekly water changes, the algae will eventually die off and the corals (if any exist) will take off. Also make sure that the bulb/s are still good. Most bulbs on a saltwater tank should be changes at least yearly, unless they are LED.
Good luck and keep us posted.