You're not doing anything "wrong"... you are actually doing everything "right". Your ferment is growing in strength over time because you're back-slopping continuously. The whiteish/grayish film on the top is the ferment. Part of the reason it's getting so strong an odor is because of the way you're doing it... with the grains in the draining basket, the mother and fermented/ing liquid below is going to just keep getting stronger and stronger because you keep "feeding it" above. Very little of it gets taken away. Eliminate the drain basket and ferment everything IN the water... don't fill it so water is "above" the level of the grain, but where the grain is the consistency of a nice bowl of grits or oatmeal... easy to stir/mix, but NOT needing to be drained or strained to use. Since you'll then be removing some of the liquid each time you serve it out, it will be much easier to maintain that "nice" smelling ferment over time.
Other than that just keep doing what you've been doing... use some, add new water and grain... use some, add more of both... stir every now and again... good to go!