I would think you could reuse it of you replace the seals but if you want to be sure it is safe, replace it with a new and/or rebuilt one.
Be sure to lubricate the caliper slide pins with the special brake silicone grease (very inexpensive) when assembling it back on your car. Check the rubber boots as well. Replace if damaged (very inexpensive as well).
Guess somebody hit the brake pedal when caliper was off, but you can clean and insert the piston. Just make certain it isn't scratched and the outer dust seal isn't broken. The piston can be cleaned up with fine crocus cloth, same a caliper bore.
Brakes will need to be bleed when done.