Help ID this algae ?

[JohnnyUngats]

I have this algae growing in low light areas, only on my rock. It appears to be gray or very light brown. It seems to stay short (all under 1 cm, mostly around 5mm long. It's soft and thin with "Y" shaped branches. It has not been aggressive in expanding it's territory, but I noticed it was able to secure a position on some nice purple coraline patches. I would say it looked like tiny gray plant roots. Thanks for any input.

 

[piranhapat]

What is your Phoshates reading?

 

[JohnnyUngats]

Hi ! PO4 is currently below 0.03 (Salifert test). I've been taking care of this 120 gallon setup for just under 2 years. The previous owner (3 years) heavily overfed the system. I'm pretty sure the rock soaked up quite a bit of PO4, so I always run PhosGuard in order to draw out any that may still be banked. I keep calcium around 425 at 9DKH. Salinity stable at 35ppt using ReefCrystals and rodi water.