A: You say the mold in question is a light brown and that it comes off easily when brushed, and that it’s not affecting the cigars. It could be some sort of plume, or bloom, but it would likely also be on a few of the cigars, too.
If it keeps coming back every few months, I’m not sure what it is, since it sounds like you treated it properly the first time. It could be some kind of mineral trace elements that are coming from the humidifier itself.
Your hygrometer and thermometer may be reading lower than the actual temp/RH, but that’s a guess on my part. If they were both high, particularly the temp, that could cause plume.
In the meantime, I suggest you try the light sanding again. Don’t use bleach this time. Instead, clean the newly exposed area with your 50/50 solution. Then, lower the temp to 65 degrees if you can, and lighten up on the RH, too. Get it down closer to 66%. Let me know what happens over a period of three weeks.
If that doesn’t solve the problem, I would consider changing the humidifier altogether, since it could turn out to be the source of the problem.
I also encourage other readers to leave comments and express their thoughts on this.