- Without an actual photo, brown crud could be a multitude of things in a aquarium, but I have a feeling this stuff is probably brown algae from too much light or nurients surfing around your tank.
- If it bothers you that much, grab your "Aquarium safe toothbrush" and scrub the area down. No need for any cleansers. The toothbrush should be more than sufficient.
Also these little b-microbes are probably growing around your heater because you haven't cleaned that section in a while.
I know once I mount a heater in a location in the tank, I might move it MAYBE once every month or so. I try not to stick my hands in the water if it's not necessary.
- The little bits that come off when you're scrubbing will be vacuumed up hopefully by the next day by whatever filter you're using.
- Be careful around the silicone edges with the brush if you have a traditional style glass tank.
I don't recall what tank/filter/heater/equipment you own, but have fun scrubbing.