You can clean the sides and "axles" if they stop all together, but don't mess with the material itself.
As long as they rotate (even if very slowly) they are fine.
When I clean my Emps., I swap them end for end. Seems to keep them more balanced and rotating evenly.
BTW, From your pic., it looks as if they are only about halfway to where they will get.
My older 2 look like dirt.(well, actually mud, since they are wet).