hey havent been on since 04 but help plz heater has brown crud on it wtf

i have two 50 gal tanks outdoors on my porch, will post pics of em, i think i allready did but in one tank i have a shatter proof stealth heater which is good but ive had 5 of them and this is the only one left working .. in my discus tank i have 2 glass heaters and have noticed that they have some serious brown gunk on them it looks almost like rust i put them in the corners as to conceal them from making the tank look like a heater depot but now i just have them hanging in the middle because the brown gunk on them is worring me they both seem to be working what is this stuff and can i remove it? im doing something wrong i recently bought another glass heater and a 10 dollar plastic cover protector thing will it get brown corrosive stuff on it as well ... any advice would be much appreciated cuz i hate it when all my fish die cuz im stupid or dont ask for help HUGZZ...