Sourdough help

Help, I have a sourdough starter I have been happily using for two years now.  A few days ago I took it out of the fridge for some intense feedings to built it up.  I discovered that once it looses its activity (is ready for another feeding) there is a grey layer (solid) on top of the sourdough.  I don't know if it is bacterial contamination - anybody have any ideas????