If it turned brown, something in there burned...likely the wiring. I wouldn't think it was safe in water any more, a little too likely to fry something - like fish, or you.
I had bought a Hydor Theo recently and it turned out it didn't work, and I had to buy a new heater - they are pricey aren't they? I got a Rena Smart heater (only one at Petland that was
completely submersible, that was pretty important to me, hate the semi-submersibles) and I had to pay $40 for it - but WOW! I have never had a heater so precise! it doesn't fluctuate by even half a degree! I'm glad now I spent the money, I want to have a
really good heater.