After My velvet issue

So after all of my fish died from the velvet about 2 days later my entire tank came up with this long stringy slime that covered everything in the tank, I changed the water in it like 4 times in 2 days i sucked up all the stuff off the substrae and tried to brush it all off the rocks while sucking the water out. Whatever this long slimy brown gunk was covered the entire tank filters pumps corals rocks everything overnight and you can guess where im going from there, Yup it Killed all my corals in my tank, all the polyps closed up and didnt open back up, the brain bleached the birdsnests bleached poclapora bleached everything died over night and i dont understand why it did that, The corals stayed the same color until i picked them up to blow the slime off of them and the second i got water flow on the corals the exo skeleton blew off like it was dust. I did this to everything in the tank and it all flaked away iv never seen anything like this before and even after all my corals in my tank are dead the stuff still covers everything in my tank. When i noticed this slimy stuff coming up i started checking all my levels everyday till all the corals died and everything was good/Normal so i dont understand what happened. Im so fustrated right now im about to give up and sell the whole set up. This is the second time i have lost my entire tank, the first time was when i went on vacation and my brother left the lights on for 4 days 'Metal Hallides" the water was 12 gallons low and the pumps were out of water still plugged in and all the clorals bleached from the top down.
   Any ideas what could of caused this brown slime all over the tank? Iv been using algee fix and all that and i still cant get rid of the stuff. It seems like when i rub it off the rocks and suck it out the verry next day theres even more. Any ideas what i can do would be awesome. Thanks

 

actually this tank has been up for a long time almost 2 years now so its not that i started it up to fast everything was doing awesome and growing fast and then i got a case of velvet with my fish and i went through the treatment process with that and then after all the fish died is when all this brown gunk started showing up through out the tank