please identify

[yellowbellied]

Cleaned out my tank this weekend and found these guys hiding in my rocks. Are they harmful?

 

[Anemone]

The shrimp appears to be a pistol, which in most cases won't be a problem, but depending up on size and species, they can be harmful to other ornamental shrimp.

The crab is another matter. There really isn't anything to give scale, but smaller crabs are usually not a problem, but most crabs will "sample" just about anything in a tank, and larger crabs can severely damage corals and even catch and kill slower moving or sandbed dwelling fish. In my tanks, any larger crab goes to a sump or tank of it's own... or into a trigger tank...

Kevin