I would just get them on a good diet, and mostly clean cage and keep the  
pouch clean.  Give it some time and see what happens.   
    We got 3 gliders awhile ago and 2 were yellow and fur not soft.  one even 
had a crusty back legs.   they didn't seem to be grooming. 
    we put them on OHPW (we feed this to all our gliders and as far as I know it 
is one of the better diets, have heard some call it high in sugar or fattening)  
and after several months their fur cleaned up and got soft.   They were also using 
baby wipes to clean the gliders, I found this seemed so wrong, to many chemicals  
wiping on their fur.  
    so just get them on a diet and clean environment. 
and a VET visit for new gliders is ALWAYS a great idea to rule out any illnesses.  
    grapes I have heard are technically fine for gliders, the thought on them is  
that once in a million something in the glider and grapes interact and the glider 
can become very sick or die.   We have feed our gliders a few grapes in the summer 
time when we get them and never had a issue, glider love them also.  
      We feed ours, on the side, happy glider pellets/cereal.  sold by critterlove 
and pet-pro.com, they are healthy and good for teeth cleaning.