A bubble is usually a prolapse. This means he was unable to poop or had an injury and his intestines came out his bum. SOMETIMES it fixes itself, othertimes it doesn't and will kill it. Put him where he can't be nipped at by anyone or better yet, block off part of the tank, give him a bit of salt and see if he can fix himself. If he can't pull it in within 48 hours, usually best to assume they wont make it.
It's also possible it's some-sort of parasite. So again, I suggest salt, and API General Cure, treat everyone. It's highly likely your other fish brought something into the tank and the whole should be treated.
Mollies also need to be in groups of 4-5+ in a 20-30 gallon tank. If that's not provided for them, they can bully each other or just plain get stressed out and depressed. This opens them up to nasties they would otherwise be strong enough to fight off. The less stress the better, but since this fella has something wrong, best to seclude him to get it figured out and treated where he can't be chewed on. Sometimes if a fish has something hanging out, the other will bite it...and if it's a prolapse or a parasite, being bitten would be very bad for everyone.
There's also the possibility of it being a cyst, or infected injury. Those can also be treated with a bit of salt or an antibacterial. Getting cycsts there isn't uncommon, unfortunately. Poor guys. ^^;
Basically try treating him and see where this goes. Salt is the best since it's currently an unknown, and mollies do love salt as is. =)