Watch the pleco, which species? They have a tendency to latch on and rasp the skin of less active fish to eat away at the slime coat. I've seen marks like that before. Often it's from the pleco eating away at the area. The stress, and infection caused by the attacks generally kill the poly.
It's highly recommended that you don't keep polys with plecos. There are a few species which have a tendency to leave other fish alone, but most keep it safe and keep the two separate entirely.