There is a lot of talk of phosban causing corals to bleach...
A lot of the talk has centered around dust from the phosban getting into the tank. So, first, you should be VERY careful about rinsing the stuff before you put it in the reactor and (as discussed in the thread posted below) wasting the first few gallons you push through it. Also, make sure that the phosban isn't being agitated too much in your reactor to the point that it would produce dust from the action of the tumbling. If you're doing that, good...
I have been running phosban on my 90 gallon for a while because I simply had an algae problem I couldn't keep under control and thought maybe it would solve my problem. In short, it didn't. Every time I added phosban it would make my corals lose almost all of their color. I never lost anything to it, but there was a clear relationship. However, I also noticed that I had almost no algae growth in the week or two after adding fresh and I started slowly but surely beating the algae back. This made clear a few things to me: 1) The phosban was definitely taking phosphates out of the water which is exactly what was causing my algae problem despite never having a phosphate reading on a test kit, 2) as soon as the phosban was used up, the algae would grow back as fast as ever, so there was a source of phosphates in my tank that had nothing to do with an overly populated tank (a six-line and a green chromis in a 90) or over feeding (I fed just a couple of times a week), 3) since I was uber-careful about the dust issue and was running a relatively small amount of water through my reactor, the most likely thing causing the bleaching was a sudden and significant drop in phosphates each time I added fresh phosban.
If you read most posts describing similar issues to yours (or mine), I think you'll find that 90% all of them will be attributable to one or both of these things (dust or sudden drop in phosphates). My guess is that the second is more prominent, since the majority of people won't bother spending the time and money on a phosban reactor if they don't perceive that they need it.
If you aren't having the phosphate issues I've been having and are just running phosban in order to keep your levels extremely low, it seems likely that your corals wouldn't react as quickly or angrily to the addition of phosban because they are already accustomed to the low levels of phosphates (I imagine this is why a lot of good SPS keepers use phosban without any ill effects). That said, since softies generally tolerate (if not prefer) dirtier water and more suspended particles (i.e., food), it seems most likely to me that the phosban is causing them to bleach in a much slower process caused by a lack of available nutrients. The different tolerances and feeding behaviors for different corals is why keeping a very healthy truly mixed reef is a nearly impossible balancing act.
Here's a thread on GFO. It might not be the best one, but it does cover some of the issues I talked about above.
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1325311&highlight=gfo+bleaching
My (hopefully) final solution for my phosphate problem:
I've now taken all sand and rock out of my tank and replaced with fresh rock (I'm fairly sure the rock was phosphate laden and releasing phosphates in to the water as fast as the algae could use it to grow). I did not replace the sand and I've increased the flow in my 90 with 2 more Koralia 4's (4 K4's, Eheim 1262 return, Gen-X PCX40 on a closed loop...and I'm still considering a K4 mod to more than double the output of 2 of those). I've not yet decided if I'll continue to run GFO since I don't think it will be necessary, but I'm considering it since I've already got it, since I have all SPS in that tank, and since (I hope) I've fixed the real issue behind the nutrient problem...
