I would think the grouper will end up eating your damsel. It seems the grouper can get to grow 1ft 8inches.
The one i seen at my doctors office was 16 inches. They require a 100g tank.
Here is a description
Panther Grouper, also known as the Polka-Dot Grouper, is an off-white fish with orderly polka-dots over the entire body that add a festive touch to your aquarium. It is found in crevices and swims along the reefs of the Indo-Pacific, where it can attain a size of up to 20". The polka-dots camouflage the fish and act as a disrupter by breaking up the body shape so that other fish and would-be predators cannot identify it.
This is a very hardy and long-lived animal provided it is kept in an ample-sized, fish-only aquarium of 100 gallons or larger, with sufficient filtration. They grow rapidly, so be sure that the tank you have them in is of sufficient size. The Panther Grouper will eat any fish and invertebrates it can swallow.