What you have is sludge...
Which is a combination of solids and paper that haven't broken down--only dissolved into little tiny particles--that have settled to the bottom, along with chemical residue, to turn to "concrete" over time. I know of nothing that will dissolve it--at least nothing that's safe to use. You're lucky that it's only about a 1/2" deep. Switching to an enzyme or live bacteria (such as our own K.O.) holding tank treatment may help to break it down over time, but it's gonna be slow.
