I've noticed it on my GSP, on the purple base at times. It appears as if the base is melting but seems to do this as it spreads. I just leave it and it ends up turning purple and new polyps coming out of it. I'm not saying there isn't anything wrong with it, but mine has done that, yet all it does is grow and spread. It is indeed a very odd, yet difficult to kill coral.
Check back with us in a week or two and show another pic.