From you description, it does sound as he may have made some sores in his mouth. Or have you left any crickets in his enclosure? If by chance there are left over crickets at night and they are looking for a meal, my guess would be that the mouth would be the best source of available alternative food for them. i hope that sentence made sense LOL. Either way most likely dried blood? Give the dragon a
bath and see if it comes off by rubbing gently with water and the pad on your finger tip. In other words don't use your nail. ReptAid can provide some antibacterial mouth aid when fed as a paste or liquid, also there is a product, I think called Rot Guard, that state it can prevent mouth rot, not only treat it. Which in any case would be another form of antibiotic you may consider. A dragon I am sure has a lot of bacteria naturally in its mouth, let alone if there are sores present.