Will this grow underwater?

not likely. if it is not already growing as a submerged plant most plants will die off in the process of converting them.

I feel like, as a general rule, if its a moss and you have to ask, it probably won't grow underwater. Or if you didn't find it under water lol

I know of some plants that are regularly above or below water for long stretches of time, but I know that even then these plants don't always do well to being submerged for more than a few months at a time.