I will write what I do before I combine non-slip tiles with dark Mapei grout, I try to discourage the customer (if he choosed the dark), customer never understands what is this and it's not their job to understand, but it's my job to do the best - in that case not to let it happen. Good luck with this
ps. I use a product called Megarens from Mapei to remove excessive grout, in your case pure Megarens-do not mix with water. Let it act for a couple of minutes, rub well and then clean well with water.