What is this growing in my tank? Doubling size over night!!

This literally grew in about 3 days. I noticed it trying to grow previously, but I knocked it all off and it came back and then I let it grow because I thought it was interesting. Then I let it continue to grow because I didn't know what to do with it and I don't want to mess up my tank cycle.

I am currently about 3 weeks into a fishless cycle. My nitrites finally dropped today ... woo hoo! but not down to zero....we'll see tomorrow. Anyway, this thing doubled in size over night. I mean seriously.....it scares me.

What is it and how should I remove it? Should I just reach in and grab it and pull it off? Is it going to ruin my cycle to get rid of it?

Thank you!

Thank you for all of your input. It really was kind of cool. These long tendrils came out of it like it was witches hair. I removed it because I think it's logical to do before it takes over the tank! I hope it won't affect my cycle. We've been waiting a long time. 3 weeks is a long time to wait for an almost 4 year old (and a mommy! ).

I will look into the excel to try to help keep it away. I see other kinds of algae growing on my other plants and rocks. You can see some of it (the brown stuff) on the tall baby tears plant behind the driftwood. I'm assuming this problem is not as bad once the cycle is set and there are fish in the tank?

Also, see the plant in the lower left? It's growing in a hole in my driftwood (it's not supposed to be planted). When I bought it, it had black on the leaves, but I figured that would eventually go away if I took care of it...but it's not and it looks ugly. Does anyone know how I can help that plant to look nice?

Thank you!