I have a melanonychia in my toe of the middle finger nail . which is lightbrown in colour. it's width is small about 0.2cm. is this harmful. i'm worried. is this cause nail cancer. i'm worried?

It is too early to tell. But it requires monitoring. I would suggest that you take some photographs on your camera now and do it once monthly x 3 months. If you see your nail starting to turn black, then a biopsy may be required to assess this pigmented area for melanoma, which can arise from the base of the nail. it is rare and treatable provided you keep a close eye on its growth/change, get chk