I seriously doubt the astrea was after the polyps. Even if it did bump into them, you'd think they'd only be temporaily closed up.
Now, the mushroom, on the other hand, is (to me, anyways), a potentially big smoking gun. I have no experience with the type of mushroom you have, but I can say that with the ones that I have, very few things are able to compete with them for space. Mushrooms show no obvious means of aggression, yet whatever they touch, tends to do poorly. Don't even think about letting mushrooms touch SPS! It's amazing how fast some SPS melt away from mushrooms. Even some of my LPS that are supposedly "aggressive" such as my bubble coral, my (now long-dead :( ) torch, have started to suffer when confronted by encroaching mushrooms. I know, I should give these guys a break, some people like mushrooms and they're perfect corals for some tanks.
The other point I have is about the spray-dried phytoplankton. I tried some of that stuff, I thought it was for the birds. I have since read that it's not really the greatest, you have to preblend it for you to get any useful particle size, etc. What a pain! I recommend you get yourself a bottle of DT's phytoplankton. You need to store the bottle in the fridge since this stuff is live
Nanochloropsis spp. (not entirely certain of the spelling offhand, and too lazy to quickly look it up right now). It is pricey but people swear by it. You can pick up a small bottle locally at Tails-n-Scales (256-9227), or you can mail-order (which is what I do, since I seem to be going through the stuff like it's soda pop, so I get the BIG bottles). At the rate I'm going through this stuff, it is a future project of mine to get a phytoplankton farm going ("Flame*Angel" over at RC has such a setup, check out her website:
Flame*Angel's Reef [and she's Canadian to boot! rrrRRRowWrrr. We need to convince her to pay us a visit over here! :D ]), but for now I'm just buying the bottles.
I have no idea if you need to dose phyto for yellow polyps though. If you think they might be hungry, why not try some chopped krill (or some chopped mysis, when the stuff gets delivered, which should be early next week, BTW, in case you were wondering about that :D ) Or some baby brine shrimp.
Hope this helps. Cheers