Looks like it's probably a sponge. I get sponges growing on the underside of my clam shells too. As far as I know, clams don't get brown jelly disease, only LPS corals.
I've never heard of brown jelly on clams either.My guess is as good as yours.It doesn't look like th sponge is bothering the clam.It does look like the clam is unusually stretching upwards.Is it getting enough light?
Sponges and clams are both filter feeders feeding on the same foods, so if you are feeding one it is not unlikely to find the other present if you also have live rock or live sand or wild harvested coral.
I've never heard of brown jelly on clams either.My guess is as good as yours.It doesn't look like th sponge is bothering the clam.It does look like the clam is unusually stretching upwards.Is it getting enough light?