The electrodes are made of silicone filled with black graphite powder. Some of this graphite may come off when you wipe them down. This is harmless because not only the surface but the entire material contains graphite.
Even if the surface becomes slightly rougher after prolonged use, the electrode will continue to function perfectly.
If you are unsure, you can check the electrode’s conductivity yourself (see: “How can I check the electrodes or electrode cables myself? ").
For large tank electrodes, you can also simply flip the electrode over—the top and bottom are the same.