This creatagus bonsai has a very nice, mature bark. Unfortunately, this leads to moss crawling through the cracks in the bark, and slowly spreading around the bark. When I just had the tree, it took up to spring the next year before I actively noticed the moss. At that moment I had lost part of the back, as this had started to rot. Now I check this tree twice a year, and clean it out completely.
I work my bonsai in different ways against mmoss on the bark. First of all, all my deciduous trees get sprayed with algea and moss remover during the last weeks of winter. As soon as I see the first buds getting really thick I load a spraying cannister with the regular street cleaning fluid, and spay the barks. I do try to not get this in the soil (Although I am not convinced this would be a problem).
After a few days, all the moss that has been sprayed starts to get brown. At this point, the moss is easy to get off your tree. So with pincers start plucking away. At this stage it is important to also look just below the soil surface. I find there are often bits of mosss there too.
After peeling off all the visible moss, I take a small brush, and paint the above-ground parts of the trunk with the pure moss remover fluid, and let the tree dry well before I water again. So far, this works well to keep down the moss growth on my tree trunks.