That's what I think, yeah. They move a little bit, then munch the ground sweeping left and right, then move a bit more. Their little mouths are like sandpaper, so they just rub away the green growth (I think, I'm not a snailologist, I just like snails and have been bit by them).

Edit: https://themarinedetective.com/2020/10/26/who-goes-there-scraping-marks-on-the-rocks/

This link isn't gonna be suuuuper accurate because it's marine mollusks that they're looking at, but you can see the similarity in the marks. The radula would be the commonality between them, which is why I think snail/slug.