Identifying a plant growing in my tank

Hi folks, need some help identifying a plant growing in my tank. It has been growing for a couple of months now. I have been letting it grow, hoping it is absorbing the nasty stuff from the water. So I am hoping it is a good plant, and not a nuisance one. I have no idea where it came from. Been growing since I moved my tank, and switched from reef to just a fish tank.

Thanks for the help.

http://i225.photobucket.com/albums/d.../WP_000331.jpg

http://i225.photobucket.com/albums/d.../WP_000331.jpg

I have that in my tank from time to time. I think it is grape caulerpa. I've never had any issues with it getting out of hand.

Ya. I believe that is it. And it is growing like a weed. It has taken over that rock completely now. I just have to trim it back, and get rid of some. So I hope it is helping clean the water