If you have a colorless creature card in your graveyard while you control Grizzled Angler, it won’t transform yet. It only transforms while its activated ability is resolving.

If a creature affected by Grisly Anglerfish’s effect hasn’t been under its controller’s control since the turn began, is tapped, or is affected by a spell or ability that says it can’t attack, then it doesn’t attack. If there’s a cost associated with having that creature attack, its controller isn’t forced to pay that cost, so it doesn’t have to attack in that case either.

Each creature’s controller still chooses the player, planeswalker, or battle it attacks. The creatures that have to attack don’t necessarily have to attack you if there are other options.