there are a number of different chalk marking products on the market- you need to check the removal instructions for the product you used. some come off very well with a damp washcloth- some set the color with water- some remove with an iron, some with a brush- some with tape-
some colors come off easier than others.
if you can not check the packaging- or site for where it came from you may want to use a scrap of your muslin & try different ways until you find what works best for you....i use a refillable chalk pencil all the time- a wet washcloth works well most of the time- sometimes i only need to brush it away- but that's the one i use...hopefully you still have the packaging for your product & can check to see the removal instructions.