What might these growths be? and removing harmful crabs.

[Youngster Dan]

I would get rid of them, or a better solution (for the crabs anyways) would be to put them in your sump (if you have one). It certainly doesn't hurt to get rid of them, especially if you are not sure if they will be a pest.

One trick to catching them is just to put a glass leaning against the rock where the crabs hide, and put some food to attract them in the bottom of the glass. Crab goes in to get the food, but the glass is too slippery to get out. Sometimes it works, sometimes it doesn't.

[NanoHuman]

xanthid crabs no doubt, eliminate them!

[untamed]

I always give the critter the benefit of the doubt before removing it. I wait to see if I can see any kind of damage before I act.

However, having had the same kind of crab that you describe create a nice dead spot in my SPS, I removed him with a pair of sharp nosed tweezers. I created a frag or two in the process.

I doubt that a crab of that type will leave the coral to fall into a trap.

As for your first photos...I agree...sponge and valonia algae. Valonia has certainly given some people trouble, but it has never caused any problem for me. I think that people get into trouble trying to eliminate ALL algae from their systems.

[gobytron]

the larger, evasive ones are likely worth some skepticism, but the acro crabs are very likely a symbiotic species and it would be a surprise if they were detrimental to the colony they have evolved to mimick...

[untamed]

Most of the nice acro crabs aren't hairy...

[gobytron]

How bad can a crab be that is mall enough to make a home and camo in an sps colony though?

what tank inhabitants would you actually need to worry about?