Cyno?

[N2theUnknown]

So some back story this tank has been up and running for about 5 months now. I've gone through the most of the uglies or atleast the worst of them. I have this gsp that has what I think is cyno growing on it. This is the only spot on the whole tank that has it growing. I've got a wave maker pulsing in the direction and have good movement params are good.
Ammonia 0
Trites 0
Trates 10 but I do have alot bound up in the starting stages of hair algae but the blenny loves it haha

I guess my phosphates could be about high I suppose. But what would cause it to only grow on the gsp? It's been like this for about 2 weeks

 

[Tautog]

Chemiclear, blue box, works great, but your skimmer will go nuts, so it needs to be watched from over-flowing

 

[N2theUnknown]

Chemiclear, blue box, works great, but your skimmer will go nuts, so it needs to be watched from over-flowing

Thanks for your quick response! I'm more so curious as to why it's just growing on the gsp? Any thoughts is it do to lack of flow and I have detritus settling in the gsp?