I think that it your are correct about the cause--the glass was starved of epoxy.  I think that this condition can only be fixed by removing all of the fiberglass and feathering in a patch, or by choosing a painted rather than a bright finish.

I have some patches where this happened.  You can see the fibreglass and there is a silvery appearance.  I decided to paint the hull.

Uh. I want a bright finish. The rest of my deck is Ok. Are these the only two solutions?

Thanks in advance

Ivan

If you want a bright finish and you don't want the white spot to show in your finish, you have to carefully cut out or sand out the white spot.  It is the only way, but it is not difficult if you take your time.  Then apply a patch of fiberglass cloth over the sanded area and apply epoxy.  Good luck.

Sorry about the Futbol.

Thanks for the advices. Finally, I´ve removed the fiberglass in that area. Now I´m cleaning the area and after that, I´ll apply a new one. 

Thanks

Ivan