Limpets are snail like, they move. Those look like little sponges. Although with blurry cam it's hard to tell. Is there a little "spout" in the middle of these or are the completely solid?
I'm with Russ & thoppa! If they are just sitting in one place, and have not moved at all I would go with sponge. Pix are not very sharp, but they look like the same ones I have in my reef. Mine have been in the same spot for the past 2 years.