Not all of the Epic abilities are spells. The ones that are have a square shape around the symbol, and I can't remember if that is the case with the Watcher or not.
However, if an Epic ability lets you cast a spell rather than an ability, it can only be used if that spell is legal (hasn't been cast, opponent isn't immune, etc).
You're right. There's a square shape around the Watcher's symbol. I had no idea that's how it works. Thanks.