It appears that icon is customizable through regular icon theming. It's called 'view-app-grid-symbolic'...
./Adwaita/scalable/actions/view-app-grid-symbolic.svg ./Adwaita/64x64/actions/view-app-grid-symbolic.symbolic.png ./Adwaita/48x48/actions/view-app-grid-symbolic.symbolic.png ./Adwaita/96x96/actions/view-app-grid-symbolic.symbolic.png ./Adwaita/16x16/actions/view-app-grid-symbolic.symbolic.png ./Adwaita/32x32/actions/view-app-grid-symbolic.symbolic.png ./Adwaita/24x24/actions/view-app-grid-symbolic.symbolic.png ./Yaru/scalable/actions/view-app-grid-symbolic.svg
It appears that icon is customizable through regular icon theming. It's called 'view-app- grid-symbolic' ...
./Adwaita/ scalable/ actions/ view-app- grid-symbolic. svg 64x64/actions/ view-app- grid-symbolic. symbolic. png 48x48/actions/ view-app- grid-symbolic. symbolic. png 96x96/actions/ view-app- grid-symbolic. symbolic. png 16x16/actions/ view-app- grid-symbolic. symbolic. png 32x32/actions/ view-app- grid-symbolic. symbolic. png 24x24/actions/ view-app- grid-symbolic. symbolic. png scalable/ actions/ view-app- grid-symbolic. svg
./Adwaita/
./Adwaita/
./Adwaita/
./Adwaita/
./Adwaita/
./Adwaita/
./Yaru/