White things in sponges are probably tunicates. Do they move?

The "inch worm" you describe sounds like some nudibranches I've had. They were about 1/2 an inch or less and looked to me like they were galloping along like a little horse. Mine were gold to yellow colored and ate only algae. I got them with a live sponge. Never did try to get an exact id, only watch closely to see what they would feed on. I tossed one in a little 10 gallon before I knew they were hamless. Some species are not harmless, so keep a close eye on it. If it is a dangerous one and gets into its host, you will have a very difficult time seeing or removing it and it may kill is quarry before you catch it.