The space slug looks fine. Why change it? It's a fantastic creature that makes absolutely no sense in real life, so if we were going for strict realism there would be no creature, period. Delete the whole scene and just say they flew out of a normal asteroid.

The only part that needs fixing are the bad matte lines on the teeth in those last shots (and maybe that strange blue effect that everyone mentioned that's now been removed). Everybody knows it's a monster mouth except Leia, so what?

If you really want to improve the scene, turn it into one of the sand worms from Dune, and insert the line where Dr. Kynes is talking about blessing the Maker and his water. ;)