What are those white thingies?

At first it was only one or two of them, but now there are more. They don't move, so I don't think it's snails. They aren't on the plants, only on the glass and the decoration. I have guppies, snails and african dwarf frogs, they are all healthy and act normally. When I scrape one of the white thingies off, they leave a tiny white ring. They are not easy to scrape off though. Does anyone know, what those could be and If I should do anything about it?