One more observation : most pigmy angels are usually 'SPS safe' on one condition - that water quality are at acceptable quality such that SPS are not stress or when RTN or STN occurs.
I have been keeping bi-colour, flame, multicolour, coral beauty, flame back, golden etc with no issues with SPS at all. The only time where they start showing interest is where a particular SPS's health is deteriorating, and when STN or RTN occurs, these angels may turn on SPS and start nipping on the SPS tissue (which apparently is a food source to them ).
Hence i believe that when SPS is healthy, the chemical slime serve as a protective coat to ward of most fishes (and most angels) but when the coating is loss, leaving the tissue that's where all hell break loose.
Note : However I have an exceptional experience with Ebli's angel which start to attack most of my SPS the moment it was introduced in the tank., even though the corals are healthy and no other fish is interested in SPS. But I cant conclude on that incident if Ebli is one of the exceptional angel that is definitely 'non SPS safe' ( because i would never get another Ebli in my life time )