If your ferts say they have mg and phosphorus, then it's probably something else. If the rusting starts off as small golden-yellow spotting, then it's probably magnesium def and you can probably add a small amount of Epsom salts to your fert mix or just put about 1/4 to 1/2 teaspoon in a gallon of water and give that separately. I've seen this kind of spotting before, however, and it was thought to be calcium def. It's hard to know unless you're absolutely positive and and not too many people really know what the spotting is telling them. Perhaps the best bet would be to get some calmag and give that according to the directions on the container and see what happens. The brown tissue is dead and won't come back but with calmag you cover two things at once, so if the problem is just of them for certain, then you given the right thing and it's unlikely that more of the other will do any harm. It's interesting that people give ferts that may not have everything you need. The grower needs to know's what necessary for plant nutrition and provide it, especially in coco or hydro grows.