I think there is more to this than just Gandalf being on the move and Tom staying in his forest. First of all, all rolling stones have had to be put into motion by someone, for the stone to be rolling it also needs to be round. (Shaped) A mossy stone isn't round because if it was it would easily move around and there would be no time for moss to grow on it. Also why isn't anyone pushing the mossy stone around? Maybe it's simply too large for them to push.
Now will moss start growing on Gandalf as well after he stops rolling around? What exactly could the moss mean, maybe it's knowledge. It's important to remember that moss only grows in dark, damp places. One could imagine dark and, damp to mean hidden places. Maybe Tom is:
Big
Knowledgeable
Hidden
Unmoveable
Unshaped
While Gandalf is:
Smaller
Younger
Known
Easily moved around by others
Not his own master
This suggests to me that Tom and Gandalf are not of the same kind. Tom must be a greater spirit. I agree with Inziladun, it's likely that Tom is a void creature not that different from Ungoliant yet maybe older. He is definitely up there with the great valas in terms of power. However if this is true one still needs to ask oneself why Tom was put into Arda in the first place.
He seems to care for the trees and for the earth. Maybe one could think of him as a gardener in Arda. Someone to keep the weeds and forest fires away? He probably took the shape of a larger fat hobbit in order to remain mossy and hidden.
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