Black Film, HELP!


New member
I have black film growing in my refugium especially on the glass, should I be alarmed? I tried removing it and doing water changes but it returns so fast.

ph: 8.1
sg: 1.025
nitrite: 0
nitrate: <0.1
dKH: 12.5
alk: 4.46
Top off and water changes with RO/DI.
TDS meter: 0 on RO/DI water