Just coming back to say it really is something you need to consult with the vet about seriously because when my vet saw what was going on, he was ON IT and stressing it was really something to start treating as in that day.
Like I said, my trainer at the time thought it was a wart on Derby, and she put some kind of (human I think) wart remover on it. I didn't even know anything was up until the vet was out for other things and she happened to have him look at my horse. The sarcoid could have actually grown into his eye (like roots from it -- my understand is not in depth but it could have been very bad), and it was important to treat it. As my vet described it to me (I was really upset), we started with a very mild treatment, this effudex (sp?) ointment, and if that did not take care of it, there were some more agressive things to do and it was serious and important. Not to scare you, but it is important to treat and treat property particularly because of the location. You wouldn't want it to cause blindness and it was my understanding from from discussing it with my vet that that was a worst case outcome (but not likely because we were treating it). I'm always asking my vet what the worst possible outcome is so that I know how serious the situation could become.
I do think the location, on the eyelid, is a key point here. Maybe it is not so serious in other locations, but the vet was really stressing that we needed to get this prescription filled right away and also, had to wear gloves to apply and it did hurt Derby as it began to work.