I have heard rumors of green star polyps growing on anything, but if I'm right in my identification than the snail pack that I received from reefs2go are carrying colonies all over my tank
It looks like it is some type of polyps. There are many others besides green. I had red in my tank until a fungus eradicated it. I wouldn't put much thought into it though. If it is a healthy snail it will be on the move and most likely the polyps might not ever open up and die off.
The purple is the mat that it lays down as it spreads. The snail would have to be in the same spot for a VERY long time for it to get glued down to the rock from the mats' growth.
On the snails that's neat , it may tear off as it grows and seed the rocks or even the glass , it looks awesome on the rear glass , just be careful with it as it spreads fast and will invade other coral.