I have 3 different colorations of polyps.. all three have the little 'bulbs' in the middle....
Are you sure they are white? If you look closely, they may actually be a very light green.. like a mint green.
All three of mine have the light green coloration in their bulbs, but one of them is VERY close to white.. so it looks white at first, if this is the case, I would say its not a sign of anything, just a sign that they are 'out to play'