It's the iron . (That was the reason for my zeovit suggestions near the start of this thread, for which I got abused)
The thing with iron is that it has varying effects of different type of corals. It's effects can dependent on previous exposure, so it's difficult to pin down a "critical" level. Plus you end up with bacteria causing more trouble. The less of them the better.
The progression chimera describes is fairly typical of iron exposure. (Search for my previous posts on iron, Saharan dust, Caribbean reefs, IronEx etc if you want more info)
I'm not sure about this discussion on adding iron. In my opinion, as aquarists we're in no position to be able to manipulate iron levels like we do other parameters (Alk, Mag, Calcium etc)
I think of iron like P (only you need a LOT less of it than even P). Organisms go to extreme lengths to sequester it from a number of sources (just like P), it is VERY tightly cycled in the system (just like P), and it tends to accumulate (just like P).
Iron in pumps is the worst possible place for it to be. I'd say that the eddy currents created in the iron with the strong roatating magnetic fields within the pump, would cause a electrochemical reaction which would literally spew iron ions into the water.
Which I guess would be why these pump parts, with the small amount of iron, (relative to say dumping a pile of rust in the form or rowaphos or other phosphate absorber) can cause much larger problems (larger release over shorter period of time).
Layton