A hairy looper. I think the loopers of Geometridae and Noctuidae are smooth, so this is probably from Erebidae. I do hope you can rear it to an adult moth to get a species ID.
Judging from where you found it, and the feeding habits of other Erebidae larvae like the Lithosiini, there is a fair chance it feeds on algae and lichens. Can you find some old twigs with stuff like that on them?
You should never leave a good caterpillar in the wild. It is a virtual death sentence. If a moth lays 200 eggs (many lay more) in a stable population, on average only 2 will survive to procreate. The caterpillar survival rate in the wild is about 1% .