Bettas (siamese fighters) flashy tails look good to nibble to many normally passive fish. hes quite possibly being picked on by one or more of the other fish. gouramis and bettas are not good species to mix as they are closely related and silly little bettas sometimes get confused.
If you can, get the betta his own small tank, as long as you keep him warm and give him plants and a cave he will be happy on his own. if you cant, a short term solution may be using a large fry net to stop the other fish picking on him (just make sure its got a solid bottom, so his tail doesnt drap down into the tank to be nibbled). but this can only be used for a week or so because its just not fair to keep a fish that size in such a small area.
how big is your tank? sounds like youve got a lot of fish in it, it could also be overcrowed or the addition of the newest fish has caused a minicycle. both of which could be upsetting your betta.
i hope this helps
Tig