Species name: Pomacanthus arcuatus
Common names: Gray Angelfish, Black Angelfish, Gray-Black Angelfish
Family: Pomacanthidae
Order: Perciformes (perch-likes)
Class: Actinopterygii (ray-finned fishes)
Maximum length: 20 in.
Minimum tank size: 150 gallons
Hardiness: Medium
Aggressiveness: Semi-aggressive with fish of different species. Cannot be kept with other Angelfish or other similar fish..
Reef Compatibility: With caution. May nip at clam mantles or hard and soft corals.
Distribution: Western Atlantic: New England, USA to the vicinity of Rio de Janeiro, Brazil, including the Gulf of Mexico and the Caribbean
Diet: In the wild, the Gray Angelfish feed mainly on sponges, but also takes tunicates, algae, zoantharians, gorgonians, hydroids, bryozoans, and seagrasses. Should be fed algae, sponges, meats, vegetable, or prepared Angelfish food.

Photo: Adult Gray Angel

Additional information:
Gray Angelfish are from the Western Atlantic region where they are found on coral reefs, usually solitary, occasionally in pairs at depth of 5 to 100 feet

This species look like thee French Angel, but with a different coloration.
Adult have a gray body with dusky blue dots on the body, caudal, dorsal, and anal fins. The face is a clear, pale silvery-gray.
Juveniles are black with two light yellow bars on body and three on head; caudal fin yellow with a vertically elongate, nearly rectangular or hemispherical black spot in middle

The ideal aquarium should have a specific gravity of 1.020 to 1.025 with a pH of 8.1 to 8.4 and a temperature between 72 and 78° F. The tank should contain large amounts of live rock for grazing, plenty of hiding places and open swimming space.

I don’t like the idea of a fish in a cup of water but this video is the only image of a juvenile Gray Angelfish I found: