I agree with John. It's hard to tell. How fast does it grow? You're right though, it's probably best to remove it while you can. If it's bryopsis, you definitely want to get rid of it but even of it's culerpa, it can still be a pain once it takes hold in a tank. Culerpa also has a tendency to go sexual and pollute your tank. That's what stops a lot of people from using it in their refugium. It works really well soaking up nutrients but if it runs out, it's going to dump a bunch of stuff into your tank and it'll look like you're growing phytoplankton.