I bought a new book and read something about java fern the other day. Since it propogates by adventitious plantlets, the plantlets develop underneath the "mother leaf". As the "mother leaf" deteriorates, this will allow the plantlets to drop off easier. I never experienced this since I usually just removed the plantlets by hand and never allowed them to drop off on their own. Also stated is that if java fern receives too much light it will turn icky but mine are under very high light and I have never noticed this. If you notice small plantlets occuring off of the roots then this could just be the mother leaf dying. If not, I would suspect a deficiency.