When I cut my big one down to frag, the base part that I left in the tank did that too, for a couple weeks, IIRC. It shed some layers of white mulm during that time, but then eventually the polyps started to come back out and it started growing again.

You really can't see the slime coat but it's there. Its how leathers protect them selfs while the molt. If it is more gray then brown it is definitely molting.