Red spot on zebra danio help plz!?

I just noticed that one of my 3 danios has a huge red spot on the side of his head. It takes up almost the entire side of his head, and almost looks like blood underneath. He's in a tank with 2 other danios (a male and a female, both larger, this one is male) and a betta, has been for over 6 months. All water parameters are normal. He was hiding behind the filter, and was extremely easy to catch. I just moved him to a different tank, but idk what it is. Any ideas and how to treat. Thanks.