I just made almost the same exact thing last night. I used the pre-made gumpaste and marshmallow fondant to make roses.
I was making the marshmallow fondant, and I used the rest of a previouslly opened bag of powdered sugar, as I was mixing the fondant, when it was hardening up, I noticed a bunch of tiny white hard pebbles all thoughout it. I figured out that, that in particular bag of sugar, the powdered sugar was (for some strange reason) full of hard sugar pebbles. So, I sifted the sugar from that particular bag and had to make more fondant, which came out smooth.
I ended up using the white pebbley fondant anyway, I rolled it out as thin as I could, and that crushed some of the sugar pebbles. Then I kneaded small fist-full's of it and that broke up more of the pebbles. I still had some pebbles, but it was not as bad.
Either make more, and sift the sugar, or, roll, knead, roll, knead until you can get as much out as possible. Luckily yours is Ivory, mine was/is blue