I doubt the decline of this coral had anything to do with you. Doesn't look like it ever had much tissue on the skeleton below the polyps before it got to you. That's kind of one strike against it. It probably got jostled around during shipment to the LFS and you're just now seeing the damage. That's one of the problems I've found when special ordering something like you did... it doesn't have time to sit in a LFS tank and have shipping damage become apparent.
As long as the brown jelly doesn't come back, I'm not sure if I'd be siphoning stuff off it. The "white goo" might have just been it trying to repair itself. Best thing going is if you have a hospital tank you can move it to. That way if the brown jelly is still there, it won't infect anything else.
While I don't think it has anything to do with the damage, your calcium is really low. I'd almost think you've got a bad test kit, except for the fact that your magnesium levels are low also... and right about where one would expect them to be if your calcium was around 300. What salt are you using? What are the calcium levels of the freshly made up salt water? As FNDsalt mentioned, you really need your calcium to be up over 400. If your freshly made saltwater calcium levels are up over 400, then you might want to start thinking about 2-part dosing (or something equivalent) to keep your calcium/alk levels up between water changes.