At least 20 of these guys were feeding on the metal grate over a pond spillway. I thought they must be associated w/the Eleocharis vivipara (bright green aquatic plant) that was rooted right next to the spillway and covering the critters, but when I pulled the vegetation up, they were actually just crawling along the rusty base of the grate, feeding on... algae I guess? The water flow is strong there, but they were holding on just fine. iNaturalist suggested caddisfly larvae (not cranefly like I originally posted!), but I can't find any that are this green color. Thanks for any help!