Why does my slurry keeping forming a new krausen?

I received some slurry from a local brewery about a month ago -- I never washed it and it separated into some nice thick yeast on the bottom and beer above it. It has been in the fridge the entire time since I got it.

Shortly after I brought it home, it formed a new krausen and I couldn't figure out why -- I figured the yeast just weren't done fermenting yet and exposure to oxygen got them started again.

I was going to finally wash it and split it up a bit tonight when I noticed it was back at it and a bunch of yeast were now at the top again, floating in a thick paste.

Anybody have any ideas why this keeps happening? Periodically I'll pop the lid to relieve pressure -- is oxygen getting in and making it reactivate? Any concern with washing and re-using this yeast?

I recently had an infected batch and the activity in the fermenter went on for weeks. It was from using suspect yeast. Although my friend (also a home brewer) tasted it last night and actually liked it I wasn't intending to make a sour ale. Although in your case it might be ok and maybe there were some fermentables left in the sample you got I would be really suspicious of this yeast and just not use it. BTW the yeast I used I made a starter with and it seemed normal enough in the starter FWIW.

Thanks for sharing. I am thinking maybe I should not chance it then... I can always get some fresh yeast another time and try again to save it. Hate to waste it, but better that than a full batch.