Ok yikes, wow

the flesh is blowing in the current and rotting. Not sure of the specific disease - as many diseases and bacterial infections can yield this result, but for now, the most important thing is treatment!
Like I mentioned to you before, you should start with Kent Marine Tech-D Coral Dip. It's a dip & conditioner that disinfects corals and invertebrates to prevent the spread of bacterial infections and protozoan diseases. Designed for all stony, and most soft corals, and you should be able to find it at any SWFS as it is mass distributed. Making the dip is super easy, and the directions are right on the bottle.
You may want to repeat the dip a second time the following day and keep it in Aaron's tank and move his healthy corals to yours if you can to try and stop further spread. If it doesn't appear to get better (keeping in mind that the already dead tissue is going to need to detach and fall away on its own as well before you can really see) then you should frag that coral and remove the diseased portion and discard it to save the remaining healthy tissue, and then dip the healthy frag into a much stronger conditioner to promote tissue regeneration. For that I would use Kent Marine's Lugol's concentrated Iodine solution. Lugol's is a very strong iodine that is designed as a coral dip to eliminate disease causing bacteria but can also be carefully added to the display aquarium as well. Directions for that are also on the bottle and easy to follow. After dipping the frag in Lugol's, you could also add 1 drop per 25gals (1 drop would be ok with a 10 gal too, jus try to make it a smaller drop).
Also make sure that you place it in an area of moderate to high current (not hurricane current, but a nice steady one), and make sure that your parameters are in check.