First time I've seen this: A Red Cabezon!

[Uncle Pug]

Could it be pigmentation like how some mice/rats are albinos?

Or maybe he just ate a ton of Crab. Or was angry another fish ate his Crab. Who knows

It is definitely a pigment thing but albinos lack pigment. I seriously doubt that this cabby was eating anything different than the green cabby two feet away. As for anger... they are all angry this time of year as they are defending their eggs... and this was the only red one.