It sort of depends on your contract -- but I would guess not. The plaster company did not install the hydrostatic valve, and they are supposed to close on their own.
Do you know whether you actually have a hydrostatic valve? Do you have a friend who is into SCUBA and has a dry suit (it's pretty cold, for a wet suit already). If you can check it, without draining that might be best. Grit can get under the lip of the valve and hold it open -- or the valve seal can simply get 'cocked' and fail to drop back into place.
Otherwise, you'll have to drain your pool, and that can be hazardous to your pool. Is your pool vinyl, FG, or concrete?
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