I think you're on the money with "it's a tick bite". That crusty, icky green/yellow stuff comes in the aftermath of tick bites.
Yes horses can have allergic reactions to them.
I know this horse has a winter coat but do your best to rub your hands all over him to check for bumps<---bumps that could be ticks. This includes under his chin, IN his ears, in his private areas and the anal area. I pulled a tick off one of my horses today
They are dangerous enough to a healthy horse (Lyme disease) but a horse that's underweight and it's immune system weakened has more of a risk.
Tick bites itch like crazy. The best thing is to get yourself a tube of WalMart's hemheroid ointment (NOT the cream) and a tube of some sort of antibiotic ointment, any kind will do.
Wipe the tick bite clean with a wet paper towel (Vetericyn if you can afford it:), then put a 50-50 dab of the two ointments on the bite. The hemheroid ointment stops the itching and the antibiotic will hurry the healing process.
It's best to do that twice daily but once daily will work if he's not at home