I've been storing my butter in my LC butter crock which allows you to keep the butter at room temperature for some time. It operates via a system where you add water on the bottom of the crock to create a vacuum seal to preserve the butter. You're supposed to change the water every 4 days. I forgot to change the water for about a week. When I opened the crock to get some butter for my dish, there was some green mold on the top. Was it due to cross-contamination or did it happen due to not changing the water? Currently I'm keeping the butter in the fridge in my crock.
Here's a link to the crock I have: