This is just practice so ignore the random icing ... but I’m wondering why my fondant gets those little white marks? It’s homemade marshmallow fondant. 
If I had to guess, I would say it is either tiny lumps of sugar or particles of marshmallow which didn't completely melt. Be sure to sift your sugar.
And use a superfine sifter. My favorite one has mesh so tightly woven that I can't actually use it for something like straining seeds from a raspberry coulis or jam - it would take way too long!
What I see happening is that the outside is drying out slightly and then when you kneed it the little hard bits on the outside are getting mixed in . I would add more glycerin to your recipe before storing in a tightly wrapped ball with cling wrap.
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