Brown slime algae on reef plugs

[plankton]

I'm getting brown slime algae growing only on reef plugs in a frag 'a rack. There is no other algae growing on any of the live rock.

I've removed and cleaned the frag 'a rack as well as each individual plug, but the brown slime just comes back. I've added GFO, carbon and up'ed the water changes but still the algae comes back. I've tried increasing the flow but still the algae comes back.

Why brown slime algae only on the plugs? :uzi:

Thanks in advance...

Scott