Where on the mast is that?
Top, bottom, middle, close to fittings?
What kind of boat
What kind of sailing?
Doesn't look problematic to me from this picture. More of a cosmetic issue.
It may be an indication that it is quite an old mast, which can be problematic if it has gone through an aweful lot of load cycles.
Insurances tend to not like masts much older than 10years, but they can last much much longer.
I'd check the attachment points for fittings to see if there is any corrosion