Brown plant ID please.

[reb]

I have had a coral frag on a rock for about 3 months. There is a brown leaf like plant growing. What is it and is it good or bad?

 

[bang guy]

I'm having a mental block and I keep thinking Kelp. It's not Kelp but it is an invasive type of algae. If you like the look then it is possible to keep it trimmed but don't let it overgrow any corals.