It'd depend on a couple of things...if the person seemed gross or nasty in some way, I might not mention it to them, I'd kind of move away.
If I was in a bad mood and things were all wrong about where I was and why, I might not mention it, (because I might not be able to deal with the person's reaction).
If I just came across a normal stranger in a normal situation I would try to tell this person, "Hey, scuse me, but you have something on your back that looks nasty...etc" and see if they were up for some help if necessary.
But I'd need to know I could absolutely cope with any kind of a response before I told them.
(Not all people are predictable!! Some do not like to be bothered. Others might be in a bad mood, or uptight, or simply generally hostile.....)