The GSP could be lifting for a number of reasons. A Few guesses....
Detritis could have built up in the affected area and rotted a hole in the mat. When this happens the surrounding area will lift.
The coral could be declining in general health in which case only an improvement in conditions would help.
Sometimes areas of the mat will overgrow itself. Then the mat underneath dies back leaving the upper portion "loose"
A combination of the above or something else that leads to the mat deteriorating will attract scavengers. I suspect that the starfish are simply eating the damaged areas of the GSP not the cause of the damage although ophiura would have a better idea than I. It would help to have a picture of the little guys but your discription is pretty good. Maybe do a search for "mini-stars" or "mini-brittle stars" and see if there is a better picture posted that you can identify yours with.
So what to do....I would use a turkey baster or powerhead to clean out the places that are lifting. Then if larger pieces come off or there is a large loose piece you could finish removing it with a new clean razor then use some of the super glue gel from your leather project to attach the pieces to a different piece of rubble rock. Place the new frag (yeah!) in a high water flow area. Not so much flow that you blow the pieces of GSP loose but enough to keep anything from settling on it.
I would also increase the flow around the original piece to keep it cleaned off. If the mat doesn't want to stay put you could (after throughly cleaning under it) use some more of your glue to resecure it.
Good Luck
SiF
PS the above assumes that your water quality and lighting is adaquate. If it is not then you should start there
