Answer:
I am the yellow hem of the sea's blue skirt.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.
What is yellow, looks like the moon and has seeds?
What lies in a tunnel of darkness. That can only attack when pulled back?
What can be measured but not seen?
If itβs information you seek, come and see me. If itβs pairs of letters you need, I have consecutively three. What am I?