How else would you describe what occurs other than 6dB/octave?

For a 6dB/octave lowpass filter, I would say power gain is inversely proportional to frequency squared (1/f^2). There's precedent for this notation, for example pink noise is known as both 3dB/octave and as 1/f.

Notice how both dB and octave are ratios, but for some reason for frequency ratios we don't use bels or decibels. An octave is a ratio of 2, a bel is a ratio of 10.

For frequency, a decade is a ratio of 10, akin to a bel. And in photography, a stop or an EV is a factor of 2, akin to an octave. https://en.wikipedia.org/wiki/Logarithmic_scale#Common_uses