HELP? My guinea pig's back is disgusting and have dirty things on it? Should I take him to the vet.?

That is so far from normal of course you should take it to a vet. I have some thoughts of what might be going on but you do not give a good enough description. Any time there is something out of norm with our guinea pigs we must take them to a vet, i only use exotic vets as they have gone to extra schooling to study guinea pigs so they know what is happening were general vets are many times guessing.