Ive noticed this thing growing on my clam since I had it (over 6 months now) my clam has never showed any signs of stress so Im not especially nervous but I want to make sure.
It resembles a worm/snail in a shell. The shell is attached to the clam and is very hard. It also seems to produce a mucus "net" to catch things.
PS: The clam is closed in the pics because I played with it, to keep it close for the pics.
Here is a better shot of the "Net"