why does my java fern leaves turn brown?

I have a 15g with t8 tube flor. with java fern attached to driftwood. My question is why after the new spiky end appear the leaves get a dark brown covering. The leaves are not dying they till grow. I have tried to wipe off brown with limited success. The tank has no algae on the glass. I have never used any supplment etheir. Thanks

No CO2 or other supplments used.
I know that java ferns use their old leaves to produce new plants. They seems to be growing fine, there is just the brown build up after some time