I have sealed my Chosera 10k , Gok 20k, and all my jnats and Nagura stones.
Mould never occurred to me though.

The Gok and the Naguras I sealed to keep them clean and maintain stamps.
I didn't want swarf muckin up my white stones.
The 10k cracked up pre sealing , I have a feeling that had I have sealed it from new it would never have cracked up.
There is no need to seal synthetics really I just like to be extra