First, welcome to AA! Secondly, try pouring vinegar over it, if it DOES fizz DON'T use it!!!! If it doesn't I don't see why not, just clean it off and watch the water parameters to make sure that nothing happens!
thanks, i also have some other questions if you happen to know why? I never see my crayfish eat, but they're pretty active. I put some food in and they leave it alone so i take it out after 30 minutes since i dont want it to decompose. I think it might be since i got them in a river and they dont like their box much. I also put the unfizzing rock in and it seems fine thanks! but they dont really use it much and mostly hide under it