I gave seen this a number of times with high magnesium levels (much higher than your level). Have also seen it and no one was able to explain it, the majority of the time the coral was otherwise healthy. I am sure there are multitudes of coral pathogens that are not commonly seen or accurately ID'd. Hope someone chimes in with more insight, I am curious as well.
This is from high magnesium levels, check your magnesium and then verify it with a different test kit.
When I seen it it only happened to one acro as well, maybe some acros are more sensitive to magnesium then others. 1450 is a high mag by the way, but when I seen it my mag was 1700.