Does it crawl around your tank or does it stay right there? The pic is a little too fuzzy for me to blow up and try to see details, but I don't see anything in that pic that looks like a sea hare. I can't think of any that are bumpy or have strings coming out of them... If you can get your camera to focus on the fuzzy thing you're referring to, I'm sure someone can ID it. You might try getting it to focus on SOMETHING, then hold the button down slightly to keep it from refocusing, and simply moving the camera until the subject appears in focus, and then take the pic. This works with most of the autofocus digitals.
Cheers,
Don