no matter what you want to use to mark your quilt you should always test to make sure it will come out---every time- it might come out of one then not out of the next---save yourself aggrivation and ALWAYS TEST.
try an oxyclean paste- brushed into the lines- or if your fabrics can handle it you could try spray & wash- or shout...
if you heat set the lines- they will probably fade with time. or the oxyclean or other spot treatment might help.

I haven't used the Sewline but I did have a similar problem with blue chalk. I discovered that it wasn't the quilt but the quilting cotton thread that I couldn't get the colour out of. It did eventually become less visible (several washes) in time but it never completely came out. I hope some of the solutions suggested work for you, it is soooo disappointing after all that work.

I'm winia's friend who had the same problem, lines (pink and also green) that wouldn't come out. I used the Sewline eraser and it didn't do the job. I've had no problem with the white Sewline. Someone here mentioned the Sewline website so I'll check that out.

I went to the Sewline website, and there is a mention that the dye is weak and it doesn't hold up in sunlight. After I
washed the quilt, I put it outside for several hours in the sunlight to dry. I am about to press it, but as I went
over it, I could not see any green markings. So, the
answer seems to be sunlight. Cross my fingers.