Asalt is heated, evolves water, and turns from green to white. is the salt a hydrate

Yes, the salt is a hydrate.
A salt is said to be a hydrate if its molecules is loosely attached to a specific number of water molecules. When an hydrated salt is heated, it loses the water molecules that are attached to it and this is usually accompanied by color change in the salt. This is why some hydrated salts are used as indicators; they bring about color change.

Less stable  hope this