The pic is kinda blurry, but it looks like a flatworm to me...google flatworms and see if it looks like that. If so, then you'll need to get it out your tank.
Flatworm exit is very effective, safe to use in your tank also. Just make sure you follow the directions, and have some extra saltwater available to perform a wc afterwards.
I always wondered that too, when i first start all of my tanks from fresh, they have some many bubbles everywhere, couldn't answer that one for you! :?
I swear I have seen something like that in my tank, thought it was some kind of creature and it turned out to be a fish turd when I fished it out with the net. LOL