OK, for it to have bleached, it's stressed by something. Did this gash appear before or after the nem bleached? If it was after, you have an underlying problem. While the lights aren't quite good enough with one down, with one left (I think the 550 has two PC lights?) and fairly heavy feeds (for a BTA) you shouldn't find that an issue just yet, in another weeks time, maybe, but I wouldn't expect it to be an issue just yet...
We need Mg, Alk, Ca and Phosphate tests also. They are all important as you are supplementing Mg and Ca. If you weren't supplementing, I'd personally only have Phosphate on the list, but if you are supplementing according to manufacturer instructions without testing, it's highly likely that you have messed those levels up. Alk is also very important (probably more so that Ca actually) to any invert that does not build a "proper" skeleton, and hence you need to know what it's on especially if you have been upping Ca, but not Alk, since Alk is metabolised in proportion to Ca...
What salt brand does your LFS use? 1.021 is far too low, I'd consider changing salt RO suppliers, or better still, mixing your own salt... Your tanks salinity is OK, but would be better higher
All the best
Rabbut