What is this red dot on my goldfish?

Hi, I've been keeping a common goldfish for about a year now. He lives alone in a planted 10gal with a HOB filter.

He's had this single red dot on his side since I got him. Its about the size of a scale, and it doesn't seem to be hurting him, but I am still slightly concerned.
What could it be?

Any help is appreciated. Thank you.