This is growing in my overflow box..

[KCNano23]

Not sure what it is but when I clean it, it just comes right back.. Kinda nervous on what it could be..

[Ashlar]

Looks like a turf / hair algae.

[KCNano23]

How do I get rid of it? It's not it my tank

[Steve]

Technically it is in your tank or it wouldn't be in your overflow. CUC and if you have any tangs are probably keeping it at bay in your display. Just clean it out periodically. I get big wads of it growing in my overflow but have none noticeable in DT.

[Ashlar]

Agreed. Keep it mowed down, keep your skimmer clean, and keep up a good water change regime. The only potential catastrophe with it that I can see is that if it dislodges all at once, it could clog a pipe. If you keep it trimmed, it just becomes another nutrient export measure.

[bbtm64]

I've gotten some in my little refugium. I pull it out and put in my tank so my tangs and emerald crabs can eat it.

[Marine_freak]

Great nutrient export!

[bkvreef]

all that said above!
If you want to stop it in your overflow put something over it to block the light in there.  I would think that could take care of it.