Yeah, they sell special cleaners to de-moldify washing machines. If you go to a drug store or a larger grocery store, look for a cleaner with the label 洗濯槽クリーナー, or 洗たく槽クリーナー. (The reading is sentakusou kuriinaa
.) I've used a couple of different ones and I think this one
, called 洗たく槽ハイター ("sentakusou haitaa") was the one I found most effective. It's a powder form, so you don't have to worry quite as much about it splashing onto your clothes.
Not sure if you're fine with the Japanese directions, but with the powder forms, generally you fill the machine with water to the top, put the cleaner in, let it swish around for a minute or two, and then let it sit for an hour or two (depending on the brand) before letting the full wash cycle continue. You'll need to clean out the little net that traps laundry gunk, and depending on how much is built up, it may be a good idea to run an empty rinse cycle again. The first time I cleaned my washer, I had gunk floating around my machine for a couple of days afterwords since I didn't think to rinse. :-/
As general unsolicited advice, different websites have recommended prevention measures including leaving the top open to let it dry and keeping the area around the washer from getting wet. (Might not work too well for those of us with washers outside...) Using bleach should help between deep cleanings. Vinegar (as a laundry softener, def. not to mix with bleach) will help a little too.