After cleaning your masturbator, it's important to let it dry properly. Gently pat it dry with a lint-free towel, or leave it to dry in a well-ventilated room.
Make sure all moisture is removed from both the casing and the sleeve. Any leftover water can cause mould to develop.
Do you see small black spots in the sleeve? Then mould has likely developed, and it's best to replace the sleeve.
Want to speed up the drying process? Then you can use special drying aids, such as a Fleshlight Air, Fleshlight Stand Dry or Fleshlight Hang Dry.