It's in a plastic fermenter that I'm about to retire. It's probably 6 years old. A couple of months back I left it uncleaned for a week and had a similar thing happen. The beer tasted fine and I drank it so no problems there. I think there are probably scratches in the plastic that is harboring whatever is in there.
This beer was three weeks in primary, I haven't had this happen again with this fermenter, untill now.
Not freaking out about just wondering what it might be.