In most planted terrariums, a mold cycle is pretty typical, especially for new terrariums. 99.99% of mold is harmless. However, mold can be an indicator of an environment being humid enough for other bad things (bad bacteria etc). This is often why some people tend to associate mold with poor health.

In all likelihood it's not a bad thing. My planted terrariums had mold pop up about a week or two after I got them put together, and then after a month or so it went away. I also dumped some springtails in there, they help decompose animal waste and also eat mold.