I don't think it's a nose bleed. Rats generally will have a reddish-blood-like substance that comes from their nose and eyes from time to time. It's called poryphin (my spelling sucks, mind). A tiny amount may be normal, a lot of rats will get some from their eyes while they're sleeping and then will clean it away later.
It can also be associated with stress. If they've been moved to a new environment, are living alone, or something similar.
It can also be associated with respiratory infections. So watch the rat for heavy/difficult breathing, poofed fur, lethargy, anything else that would indicate the rat is sick. If they show any signs, they need to go to the vet.