It's sliming.
Some SPS do it regularly to get rid of waste, or when bothered by water conditions, another coral, or you.
I've found that some of the corals that slime a lot need to be given more room than other SPS, because they'll quickly kill another SPS that gets too close. I had a green slimer (understandably one of the slimiest corals I have) fall close to a green digi frag, and that frag was a skeleton in a day, even though I separated them after about an hour. (Fortunately it wasn't anything important, and I can just cut another frag from that quick grower.)