There will be precipitation over time, yes, and moreso with rockwool (mineral fiber) than with proper rigid fiberglass. Personally I would never use rockwool for a cloud given the choice, but from past experience will always add a layer of 'ultra' thin plastic film under the final fabric in the case of overhead absorbers - particularly over the desk regardless of wool type.
Hanging the cloud from drywall can be easily accomplished by using hooks with drywall anchors - the cloud then suspended from these hooks with chain, wire or strong cord.
No trees were killed in the sending of this message. However a large number of electrons were terribly inconvenienced.