if its bryopsis, you may have a war on your hands. i made the same mistake, it looked really pretty and wavy at first, and my GF said to let it go, but then it became very difficult to keep under control. you should check the link recommended by gary, and make a possitive ID on this stuff, then folks here can direct you better, if it's just chaetomorpha then no problem, just pull some out when it gets overgrown.