Have you recently moved it or re-arranged your rocks? If so, then put it back to its earlier spot and see if it improves. Otherwise, if you haven't made any recent changes to the tank and have only recently noticed the polyps not fully extended then I would look for an explanation other than lighting. I have a birdsnest in my 90g, and also had one in my 10g, and found both to be very sensitive to parameter changes. I ultimately lost the one in my 10g due largely to temperature fluxuations. The one in my 90 was one of the corals that retracted its polyps when VOCs from carpet cleaner made their way into my tank, but it has recovered since then. I'd check all of your parameters, make sure your temps are stable, and make sure no one released any cleaning chemicals anywhere near your tank. If everything checks out then do a few slightly larger water changes and give it some time to bounce back. One other thing to check is salt creep on your light - while I highly doubt this is light related, if you've got major salt creep on your light, then its intensity will be reduced and your coral wouldn't be getting as much light as it normally does. Otherwise I don't think your light is to blame :)
