Sorry for spoilers, but this is important:

I rescue the Moth Priest, but as soon as the barrier is dropped, he attacks Serana...after he is beaten into submission, I can have a conversation with him, but for some reason, whenever he is 'healthy' he attacks people.

This makes it extremely difficult, if not quest breaking, to actually have anything to do with him, as he attacks all the Dawnguard, and only grovels around on the floor after he gets hammered. I semi fixed the problem by wandering around behind him casting harmony spells, but now the interior of Fort Dawnguard looks like it's been hit by repetitive tornadoes.

Is there a fix that anyone knows?

I had this same thing happen on one of my test run throughs of Dawnguard.

For me the issue seems to have been a glitch in the "Prophet" (Hunter) quest progression stages. In Forebears' Holdout the quest would automatically progress from "Rescue the Moth Priest" to "Speak to the Moth Priest" while I'd be in the middle of fighting through the thralls to get to where the Moth Priest was being contained. I noticed the quest progression because it would automatically put my weapon away for me even while I was in the middle of fighting the thralls and vampires! Not good timing. I could finish the quest seemingly without issue (if you don't count only being able to talk with Dexion while he was recovering from attacking Serana) until I returned to Fort Dawnguard only to find Dexion fighting all the Dawnguard members there. It later broke the game in that I couldn't progress any further as Dexion was always "busy" while recovering between fights with the Dawnguard members.

I "fixed" the issue by editing the location of the automatic quest progression triggers in Forebears' Holdout. I moved both the one for the Hunter and Vampire sides of the quest so they are pretty much over where Dexion stands and moved the vampire spawn points a little closer to the entrance of the cave. Since my edit I haven't had that weird quest progression thing happen with three subsequent runthroughs (2 Hunter, 1 Vampire).

As for a fix for your game - try re-doing the Prophet quest and try to make sure it progresses correctly. That seemed to be the issue for me.

Edit: Did another run through Prophet last night and it also progressed correctly, so that makes three test runs after my CK edit.

Thanks muchly.

I progressed throught the storyline simply by using harmony spells in fort dawnguard. I may simply put him as non essential and rub him out now...he's a bloody troublemaker.