Melted chocolate wafers
Chocolate turns white due to "fat bloom", that is, the fat content of the chocolate wanders to the surface and crystallizes into this white bloom that you observe. This can be avoided if you cool your chocolate in a cool, but not cold spot. 18 °C or about 65 F seems to be the sweet spot for this. Source here.

Alternatively, you could try to reduce the fat content by using chocolate with a lower cocoa butter/fat content.