GAH! This is my second reply that AA has eaten!
Ok, to sum up the previous ones..
As far as the glass goes, he may just be acting territorial - very common for Yellow Tangs. Ours waves his spine at us (white thing between the body & the tail) every now and then. We call it "The famous Tang-butt dance". He hasn't figured out that he isn't good at killing us yet. But who knows, maybe with practice :]
This behavior seems normal to me.. seems like he's done acclimating and this is "his" territory now. How big is the tank?
The red is likely caused by a bacterial infection.
I suppose I should have asked this earlier, but what else is your water like? Tangs are very sensitive to water quality.
I've heard of low PH
doing it. You'll want to get that up, but slowly. Quick changes don't usually make things happy.
I wouldn't worry about him dying just yet.. just keep that water quality good, watch for other signs, and make sure he's eating a good diet.