This is a very small hag moth larva; it will get bigger in time. It is difficult to imagine that this is a caterpillar! This one appears to be missing an arm on the right side. However, Bug Guide says: "Bizarre, brown, hairy creature that resembles some sort of aquatic creature more than a caterpillar. Three pairs of long arms and three pairs of short arms, which are "deciduous" - often one or more is missing."