It migrates up from your underwear, so maybe that's why mosh doesn't get it, she goes commando?
From
http://en.wikipedia.org/wiki/Navel_lint
Navel lint consists primarily of stray fibers from one's clothing, mixed with some dead skin cells and strands of body hair.
Contrary to expectations, navel lint appears to migrate upwards from underwear rather than downwards from shirts or tops. The migration process is the result of the frictional drag of body hair on underwear, which drags stray fibers up into the navel.
Women experience less navel lint because of their finer and shorter body hairs. Conversely, older men experience it more because of their coarser and more numerous hairs.
Navel lint's characteristic blue-gray tint is likely the averaging of the colors of fibers present in clothing; the same color as clothes dryer lint.[2]
The existence of navel lint is entirely harmless, and requires no corrective action.