I found some blood on one of my gliders nails this morning, but no one seems harmed. Should I worry?

Blood is always a concern. Go over them really well and look for anything you might not be seeing under their fur. They could have been scratched or poked by something in the cage or may have had a little too rough of play in the night. Possibly a nail was snagged on something and in their struggle to get free they tore the nail exposing the quick. Keep an eye on it and check everything in the cage for sharp points as well as any fabrics. The safest fabric to use is fleece which is much less likely for them to get caught on. Sewing stitches also need to be carefully watched to make sure seems are not exposed.

Does he need a nail trim? If his nail tip is needle sharp he might've snagged it, even if you're using fleece bedding and proper hidden seams. If it's only on the one nail I would think a snag is most likely. That happened to my glider the first day I got him, he tried jumping out of his pouch and caught a claw in the fleece. When I checked his foot there was blood almost embedded at the base of the nail. It healed up quick though and he didn't seem to notice it.

Keep an eye on it, maybe wash off the blood with a wet paper towel. As long as he doesn't bother it it should heal up fine.

And are you sure it isn't anything you fed him? The first time I fed strawberries I nearly had a heart attack seeing my baby's chest with the red staining...until I remembered what was for dinner :)

Sometimes dried red fruit, cherries, watermelon, raspberries, blackberries etc can stain a bit if they haven't properly cleaned up.