I have encountered moldy kefir twice.
The first time was with my water kefir. I often use my extra kefir curds to culture things like apple juice and sweetened hibiscus tea. But this is a slow process compared to milk, and between batches, I just put these curds in sugar water and keep them at room temperature for several days at a time. Once, after I hadn't changed their water for at least a week, black mold grew on top of the curds--only where they were touching the air. The water kefir tasted bad, but I decided to keep it around and change it more often to see if it would recover. The kefir curds seemed to remain healthy, but the mold did not disappear and the water kefir continued to taste bitter. I eventually threw it out.
Within the past week, my regular milk kefir--which I have been keeping since September 2011--has had a very light white mold on top. Again, it only grows where on the surface where it touches air. I have not yet tasted a batch of this moldy kefir. I have three batches of it on secondary fermentation and am continuing with the kefir as normal. I am hoping that the mold will stop showing up, but I fear it has become part of the kefir culture since it has happened three times in a row. I like to think that any foreign yeasts or bacterias that could manage to join the stable kefir culture must be harmless...but I don't know if that is really true.
I know that with kombucha, mold often grows on top of the pancake where it touches the air. This is normal for kombucha and is not harmful. All you have to do is flip the pancake over for the next batch and the submersion will kill the mold. I wonder if it is the same for kefir culture.
