I'm doing a soccer cake that needs to be finished tonight and I was going to pipe some buttercream grass around the top of the top cake so that it goes around the soccer ball however for some reason when I coloured my buttercream green it has white flecks all through it. Is there anyway I can fix this and why has it happened? Now I'm thinking I will just colour some coconut instead but how will I get that too stick too the fondant? Do I just spread a small amount of buttercream on top or what?

Thanks so much

You can either spread a small layer of icing on your fondant or I would just moisten the fondant with water to make it sticky, Then top with coconut.
Either way would work.

I have done many cakes with coconut as grass. Spread a thin layer of green royal, (soft peak,) icing over board, then colour your coconut green and sprinkle over wet icing, as much as you want. it really does look like grass.

Greetings from Hull England

Or you could use piping gel.

To do a quick grass, I just spread out the green buttercream and use your spatular on the top to make it look tacky. Just tap your spatular all over the top of it and make it stand up a bit.

Sounds like you probably have clumps in your frosting where your ingredients didn't get mixed all the way through. Probably too late, but you could always try throwing it in the mixer quick and trying to get the clumps out