I have two gnomes, one returning to the garden from our brook with a string of minnows, and another riding a turtle across the same garden. They share their garden with a ceramic toad and a cat basking on a flagstone before a solar powered fireplace amidst the daylilies.

At the end of summer I wash up my gnomes with soap and warm water, dry thoroughly, and then touch them up with acrylic craft paint if they are faded looking. Mold and lichen haven't ever attached themselves to my gnomes or other critters, but if it ever does, I will probably substitute a 30 minute soak in a clorox solution for the soap and water wash. Then rinse, dry, and paint. The bleach should kill and remove all fungus/mold/mildew. Your gnomes will love their trip to the spa!

(This was from a couple years ago. My gnomes and critters disappear under a sea of foliage once the plants begin to flower.)

Maybe this video from the Utah State University Extension can help? (Probably not, LOL. It could be funnier, but you can't expect too much from a government agency, I guess.)

Here is a link that might be useful: Gnome Management in the Garden

That video is very helpful. I don't have that particular invasive right now, but you never know when it could just sneak right into the garden!