oval bug things


Reefing newb
so on the side of my glass, in the sand, there are these little oval like things that look like that gray looking bug you would find under a rock. my question is what are thez, i think there some sort of pot or maybe sand dollars, are they harmless, and what do they do ?
i looked up a picture of them and it doesnt look like them. maybe egg shells
but there inside the sand like a few cm from the top and are attached to the glass i tried pulling one off but it was like suctioned onto it
ill try to upload one when i get back from my trip
but the closet thing they remind me of are those little gray bugs you find under rocks they are like the two twin bugs from bugs life
They are good. There are tons of types of pods, and it would impossible to identify each one we have in our tanks. The pillbug looking ones are pretty common.