Anyone know what this might be?

It's hard to tell where it is. More info?

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.

That is just yeast stuck to the walls. I would soak that bad boy in oxyclean free overnight and keep right on using it. That is 100% normal.

I'd get another bucket, if was me. Isn't worth wasting a batch. But, really? You left if dirty for a week? Dude.....

If your bucket is 6 years old, it's time to replace it.

I'd replace too. Maybe upgrade to a 6.5 gallon glass carboy.