If I'm looking at the right thing in the photo, the green stringy stuff will be a type of algae. Might be Bryopsis or a related species since one bit looks feathery (but not thick enough to be Caulerpa), but it's hard to tell. Unless you have something in the tank that you know will eat it (sea hare, certain sea urchins, etc.) it's usually best to remove it so that it doesn't become established as a pest, as many regular grazers like snails and grazing fish won't touch it.