The minis use two systems, GPS and the downward Visual Positioning System (VPS). They are used together in combination in varying amounts to control position. The VPS is relied on more for precision accuracy at low altitudes (below 30m??) then the GPS takes more control as the mini rises.
As
@PhiliusFoggg said when light conditions are low, the optical VPS isn't working as it normally does at low altitudes and is forced to use the less accurate GPS signals to hold position. Without the VPS to accurately hold position the mini will drift slightly. It's in a mode often called GPS-ATTI where the mini will coast to a stop because it can't optical see the ground, and as you say, it will feel a bit slippery rather than quickly brake as we usually expect.
There isn't a problem flying in ATTI mode just be prepared for it to slide and drift a little. You won't notice much difference at higher altitudes as it will be using just GPS and will behave normally. As the Mini's bottom Landing sensors don't use visual light, they will act normally in dark conditions and aid in slowing a few feet above the ground when landing.