Take a pic of the crab to help id

http://www.wetwebmedia.com/Arthropod...491283@aol.jpg
it looks like this.

If that's the same one then it was ID'd here
http://www.wetwebmedia.com/corldisfaq7.htm
It is pretty much reef safe and actually helping your coral according to where you took the picture from.

You want those guys. Acro crabs are a good crab to have.

looks like alk burn to me , check your test kit

That is a commensal acropora crab, they don't harm the coral, just live within it. A crab wouldn't be doing that kind of damage anyways, there is no way it would expose itself on the top of the coral to snack on polyps at the ends of the branches when it can remain hidden inside the coral and do its damage there.

That damage is either light related or flow related, guaranteed. If you have moved the coral lower or out of direct flow you should see the tissue recession stop immediately and should regrow (or at least start to regrow) over the affected areas within a couple of weeks.

Thanks everyone for your comments. Appreciate it.:biggrin:

As many have said it may be alk burn, although are any other acros in the tank experiencing swelling at the tips? the flow may have an issue aswell, I would chop the burnt tips off and try all the things suggested... It should recover.

I am with Lance on this one a few of the tips look like there is some algae on them.

So if it is algae do I scrub it off? Because my other coral is fine.