crab eating coralline

[rt67ghy]

I have a crab which comes out at night and eats some of the coralline algae from my rocks. I can't get a good enough picture for an ID but anyone knows what crab eats coralline?

 

[Betta132]

Probably an emerald or related crab, but a pic would be very helpful.

 

[rt67ghy]

I'd be happy if it's an emerald and I'll certainly post a pic when I can get one. The best pic I have so far is of only one leg. It's very secretive and I only see it once or twice every few months.