I believe it's just the mouth; sometimes, when my mushrooms are agitated, five tiny arms, each no longer than 5 millimeters (half a centimeter) come out. I don't know what the tiny "arms" are, but I have concluded it's natural as I have seen it in others' tanks. Sometimes the "arms" in my mushrooms just poke out of the mouth, giving the allusion of a dot.
I hope this was coherent....