debris settling at the bottom of pool and spa

I just bought the house last week. It did have some debris on the side walls and I brushed them towards the pump hole thing but they haven't went away now they are just settled on the deep end.

The only thing I can think of is I need to add DE to the filter because I backwashed and later found out I need to add more. I don't know how to work the equipment fully yet, so I have Leslie pools coming this Saturday.

28.5 FC
0.5 CC
80 TA
750 CH
110 CYA
7.6 PH