The pic is too small to see the mark you are talking about. It may have been resized in the process of posting it. Can you post one that's larger? Perhaps more than one to give a few views? Thanks.
Add pellets to the list - the food dyes in them can color the beak, too. I've also had red pellets cause poop to become pink or red and it looks like blood. If I accidentally get colored pellets I pick the red ones out now.I can't see the picture, it might be from something the little guy is eating. What is their diet? My birds get stains from their food, like green from their spinach and orange from their carrots (which always looks hilarious, it makes them look like they're wearing lipstick)