It's actually my large Tyree green polyp toadstool that is especially finicky. Got it from another local reefer, and it took about a month to really acclimate and open fully (it also shed during this period). The polyps would kind of peek out, but never extend fully. Ultimately, I found a spot for him on the bottom with high flow and left it alone. Finally, it came around and now looks great. My other toadstools were opening with 30 minutes of being put in the tank and aren't nearly as persnickety. Mine gets a blast of water every 7 seconds or so... it takes some serious flow, so you may just need more flow. If the polyps are at least sort of coming out, that's good sign.
The other thing could be that the nitrate is actually too low. Sometimes, having just 1 or 2ppm can help to perk things up, but if other stuff looks good, I wouldn't think that's the issue. With ULNS, pale colors are often a sign that the water has been stipped too much.