Hitchhiker ID
 

Actually its not a hitchhiker as much as new growth. I had a flame scallop die off about 2 months ago (and I dont need the lecture on thier captive survival rate). I left the shell in and it has developed a balloon, it is purple in color and looks just like a balloon. The string part is just thin purple (rigid it stood up when pulled from water) and the balloon part is just a purple bubble and it is transparent just like a balloon. Anyone have any idea what it is.

Have you touched it? Perhaps either a bit of cyano with a lot of bubbles underneath, or an odd specimen of Valonia sp. algae. Or maybe some kind of tunicate... have you got a photo?