He is one of the younger babies (not baby long ago) and Bella's boyfriend.

I saw that top of his head (where yellow crest is) has some blood residue. I spread out feathers and saw that he had some kind of wound on top of his head that is covered with feathers (nothing terribly looking from what I saw but he was kinda wiggly and didn't let me see very well). His behavior seemed to be pretty normal.
I was gone part of the evening and didn't see how it happened.
When I was looking at it, the wound wasn't bleeding, just some dry blood. I didn't want to dig deep into the wound

Can it be dangerous? I can only imagine that he had a fight with somebody (which is strange, right now my birds are in a rather sleepy mode, not hormonal at all). Or could he hit something ?

It's possible that he flew into something, or that something fell on his head. If he's acting normal then everything is probably OK. With wounds there's always a risk of infection, but the risk is pretty small. If there's a scab on the wound it's already closed off, and there's nothing else to be done about it.

thanks, I think he is all right.

Could also have been a little pin feather that's been caught on something. Seen it happen a few times with the budgies!