That's just joint compound someone stamped on the ceiling and failed to knock it down like they're supposed to. Normally you stamp it, let it dry for a short time, then come back and knock it down with light pressure using a circular motion to blend the texture a bit and make it look good.

I had a room in my basement like this. I used my 10" mud knife to just scrape across it and flatten all the really pointy parts off so it was close to how texture is supposed to look. Then I used a wet rag to soften the edges, let it dry and then repainted it and it looked a lot better.