The ones on the Martelli mats are mostly accurate. I use the mat as a rough measure and to keep the bottom of the fabric straight.
If it's just from cleaning then you probably can re-mark. If it's from cutting, use a different spot until it's time to get rid of the mat.
The light blue lines seem to leave residue on a cloth when you clean it. (I've got overspray from 505 on my mat)
Even just plain water seems to pick up the light blue. I did find one brown fabric marking pen that put a mark down that won't come off without a chemical. I'd bought the pen many years ago for an autograph quilt and it still has ink. I was marking lines to practice FMQ when I accidentally touched the mat.
This pen did not bleed and the mark isn't coming off unless I use a solvent. It's made by Stabilo and any good art store should have it.