What is this?

I'd not hurt it. For it to just appear means your tank's environment has come into alignment with its needs and that there is a biodiversity niche to be filled. If it starts trying to take over something else then I'd manage it some way.
 
Thanks, but yeah, obviously some type of macro algae and you probably won't be able to get a solid ID on it untill it grows out fully. As others have stated, something to watch though as such algae, without any herbivores that control it, can become a pest.

Chuck
 
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