Yellow sounds odd; if it looks white it could be mycoderma.

If it's forming a film get rid of it; I would put my thumb over the
air-hole in a wine thief and slowly insert it with the goal of
overflowing the carboy and washing the film away. Then I would sulfite
as Tom suggested. It can't be something that should be there as I see
it, try to get rid of it.

Here is an article on mycoderma, but again I have never seen anything
yellow grow on my wines.

http://www.oardc.ohio-state.edu/grap...an/van0199.htm

Joe


Joe