What Is This

It looks like Botryocladia. I have some in my tank. It's a slow grower and very decorative.
I have not found it to be the least bit invasive.
I had a large amount of it at one point that kindof turned into a detritus trap,
but i just pruned it back to the base when that happened and let it grow up again.

Here's a picture of it before it grew so big that i had to cut it back...personally i think it's an awesome macroalgae

Botryocladia.jpg
 
Yes thats exactly what it looks like under water and it is very slow growing in three months thats how much its grown just didnt know what to do with it.
 
I bought mine from ebay, and put it in my refugium. It died pretty quickly, and then over a year later this bit sprouted out of the rock...I was so excited. I think it's like caluerpa in that once you've had it in your tank there is always the potential for it to spring up somewhere. If it were me I'd do my best to keep what little bit of it you have alive till it gets big enough to tie it to a rock with fishing line and hope that it roots.
 
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