have you tried washing the quilt?
visit the web site of the maker of the pencil you used and see what they say to remove the marks- or check the package--and hopefully you have not done something to set it permentently already.
read somewhere --- erasers work to get pencil marks off- i would be (timid) with having to do that much rubbing- if its a large area- but i would start with the manufacturer and see how you are (supposed to remove it) then from there the quilt would go in the washing machine...
good luck- and ALWAYS TEST for ease of removal BEFORE marking any quilting....even if you used this marking item on your last quilt and it came out fine---test it on the next one...it might not work so well next time--some fabrics seem to (hold -on ) better than others.