This tank already looks very natural, with some algae growth, bio-film on branches.
I agree with
Stanzzzz7
-, it is at the substrate level that this naturalness is a bit lost. Add leaf litter, small sticks, maybe some pods,cones, etc... as break down will have a much more natural feel to substrate.
Also, possibly try building some of these mixed textures (of leaves, twigs, etyc... in small areas right by branches, in between rocks, like they have accumulated there, layered over in time. Add some leaves that are green, some brown, this will add color as well.