Green slimer question

[ctrulock]

I have a small frag in my tank way up top of rocks about 18" total (10 inches under water)off of 150w 20k light. Today I ran my tank without the acrylic top and 1/2 of my slimer turned a redish brown. I then put the top back on and it seems to be getting better (green). Could it be getting too much light? 18 gal cube tank.