Not good, prolly red acropora mites, they are arachnoids that feed on Acropora tissue or mucus. Although they don't always kill their hosts, they do weaken the host Acropora. This may be evidenced as an initial loss of coloration in the colored portion of the tips (not the white growth tip, but if your tips are a lighter color than the rest of the cral tissue and it turns white, uh oh!) Usually seen in blue and purple acropora for some reason.
There is a treatment for confirmed cases, but it may kill all the crabs and related crustaceans in a treated system. Get a magnifying glass and look closely at the spots. I have a picture or two here somewhere, but I cannot locate it right now, check RC in the pests forum for SPS.
sorry, but this sounds like what you have. Another reason to keep a
quarantine tank set up, as prevention is literally worth a pound of cure in this instance.
