Erwin gives him a bit of a quizzical look at first, a little confused by the reaction, but then fishes a handkerchief from his pocket and looks around. At first, he considers just wandering into the surf to get it wet, but then remembers when he was exploring it earlier how salty the water was and that it left kind of a funny feeling on his skin, so maybe not that, if Levi is complaining about feeling sticky.

Back near the vendor's cart, he spies something that seems like a pump...? but then sees someone walk up and press a button to shoot a stream of water out of a spigot and lean in to take a drink. That'll work.

"Wait here," he says to Levi, and trots off toward the little pump (drinking fountain). He returns a minute or two later with part of the cloth damp, and holds out his hand for Levi's.

"Here-" and if Levi lets him, he'll gently clean it away.

Alternatively, he could just snatch it from him.