Clear bubbles on soaked rock

[A3quatics]

So I’m getting ready to setup my IM 25 lagoon. My rock was once live (2 years ago) and I placed it in a 20 gallon brute with rodi water with the heat cranked to start the decay of dead matter. I went to rinse of the crud and these little clear bubbles are attached to the rock. They’re firm and filled with fluid. Anyone know what it could be?

 

[EeyoreIsMySpiritAnimal]

Looks like bubble algae to me

 

[A3quatics]

Looks like bubble algae to me

That’s what I originally thought but second guessed myself cuz it was cooking in complete darkness. Thanks