This has just started to appear on some of my rock. On this particular piece there are about 4 of them. The have a purple base and tentacles. There are also two brown lumps at the sides which you can see at the 4 and 10 o'clock positions. Thanks for the help
This is either a Corynactis or Pseudocorynactis spp., related to corallimorphians. In my experience they will reproduce slowly. Usually they don't expand like that during the daytime.
