The crabs are probably just normal crabs that hitch hike on SPS all the time. I have at least 3-4 in my SPS.
Without seeing better pictures im going to go out on a limb and say your probably dealing with STN (Slow Tissue Necrosis). Unfortunately there is nothing that can be done about this and you can only cross your fingers and hope it stops, or frag the coral and hope for the best.
Search RC and This site for "STN" and "RTN". It's one of the most popular topics with SPS husbandry.
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