I'd give it time. What lights do you have it under? And was it brown when it came in? When I upgraded to my 40b my slimer browned a bit but it bounced back pretty fast and is extremely green. I have it at the top of my tank in high flow.
Sent from my iPhone using Aquarium Advice
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"