The waterfall is a graphical display of the energies in each frequency bin resulting in the FFT of the sampled stream. Whether the demodulation code uses an FFT or not, no energy in a bin means no energy detected means no signal to decode.
You may be confusing the limited dynamic range that can be displayed using brightness and colours. That can result in the user selecting the “brightness” so that strong signals do not “washout” the display and weak signals are not displayed. If the dynamic range of the input is say 60dB and the waterfall only has 40dB of display range, then setting the display so the biggest signal produces the max brightness will mean signals in the 0 to 20dB range being invisible.
Solving this problem is difficult, it’s why you have accumulate modes so the signal on the waterfalls are time averaged so weaker signals stand out more. This code is very CPU intensive and so sucks the life out of your phone battery quickly and is missing from lots of phone apps for this reason.