The trouble with that is it heats up the rocks that it's touching, which heats up that spot on the heater. Uneven heat dissipation is what can cause a heater to crack.
The heater should have a rubber/plastic bottom-cover. It's OK for that to touch; and the top is OK too.
All that advice is for glass tube type heaters. I just looked up your heater. It has a black plastic case which won't crack. I wouldn't put it under the gravel because you don't want to spot-heat your bowl. But laying it on top is not a problem.
Clean the white stuff off and try again. Tell us if it comes back.