Most of us have seen something like that at one time or another. It's just that most don't share pictures of issues or problems so it's hard to get knowledgable about problem solving. I think it would be great to have a dedicated section with problems and solutions assuming anyone could accurately identify a problem with its solution. If we could though, it would be a big improvement in reef keeping
My first thoughts are usually shading. The only real reason I say that is then you don't have to take responsibility.
I dont know anything about rust causing specific stn but I'm interested. Certainly a review of the tank's parameters makes sense, don't know how consistent you maybe with testing or not. And as has been said by many others, your parameters are pivotal to success. As a side note, actually everything I say is a side note, it's interesting to see and perhaps discuss the changing thoughts on parameters from high alk a handful of years ago 10-12 dKh to current thinking of 7-8. It's even fascinating (to me at least), the change in nutrient issues, from too much nutrients just a handful of years ago to the too little nutrients of many reefers--what changed, and I'm not referring to carbon dosing whatsoever.
Flow is an issue as well. It's kind of like shading in that as the tank matures with coral growth even with power heads, flow can get diminished and inadequate flow doesn't allow cellular exchange to take place and the coral will have problems.
I don't think it's pests either because when it comes to pests I think of aefw and there are no bite marks here
Sorry. I don't think I added much by my rambling. But I'm glad you presented your issue with pictures. It is a subject worthy of much discussion