It's probably really heated water that's come in direct contact with the heating element. I'd get it replaced ASAP, water's certainly getting inside the heater in contact with electric powered components.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.