Coral Colony is a uncommon Creature — Wall card from Dominaria United (DMU), collector number 45. Current market price: $0.25 (foil $0.20), TCGplayer market prices via Scryfall.
Defender {1}{U}, {T}: Target player mills X cards, where X is the number of creatures you control with defender. (To mill a card, a player puts the top card of their library into their graveyard.)