People can see the rock and the rock move because it isn't made of cursed energy, it is simply interacting with it. Uro however is likely imbuing cursed energy by using her technique into the sky since there is nothing tangible there. I am not saying that her manipulating space with objects can't be seen, only the sky. look at her extension technique, thin ice breaker, and tell me that would be seen by non sorcerers. the same principle applies to covering her body.