Refugium gunk

[Mrt88]

Hi all,
I have a decent size fuge in my Red Sea reefer 450. Its lit by a Ai prime Reef 12 hours a day. I am getting this weird film of funk around the top of the cheto every time the light is on.
I have it reduced down to 20% power and still the same.
the few days I had the light off no funk.
Any ideas what it is ? Should I be concerned or is it a lack of flow ?
Thanks all

 

[Spare time]

Try and get a pump to tumble the chaeto and see if that happens. Do you think it could just be dead chaeto out of the water?