Worms truly do eat the products of decomposition. They do not eat the food. They eat the mold, fungi and bacteria. Yet an overgrowth of these items can be problematic.
If you have an overgrowth of any of the above, it is likely an indication of overfeeding.
These organisms pose no threat to the worm bin or to the garden. However, they can pose a threat to you if you are allergic to any of them. I am extremely sensitive to mold spores, therefore, an exposure to a moldy bin sends my into a massive sneezing episode and requires immediate attention. After I have gone outside away from the bin for some fresh air, I return to expose my bin to 30 minute of direct sunlight, then I leave the lid off for 24 hours and do not feed the bin for 5-7 days. In extreme cases, I have been known to remove the top layer of mold and throw it away. (gloves and mask recommended if you do this)