He does sort of snatch it, but possibly along with Erwin's entire hand as he leans in to scrub the damp part of the handkerchief against the corner of his mouth before letting it drop lower. At that point Erwin is allowed to wipe his hand with the rest, though if he's let go by then, Levi will just do it himself.

When he glances at the other boy again, he's pouting a little, but it's not angry. Maybe more like a little embarrassed and a little surprised, as if he's just seen him in a whole different light. His breathing seems to be evening out, too, and he doesn't look panicked anymore as he looks to the side and sort of kicks one foot against the sand.

"...Thanks."