Thats not hair algae on your java fern, its roots. Little baby java ferns grow on the tips of the leaves on java ferns. Its how they reproduce. First the roots appear which kinda looks like hair algae, then the little leaves appear. Then the main leaf will die off. Then you can remove the dead leaf, pull off the the baby fern and replant it.So unhealthy plants leach ammonia ? such as old raggy java fern leaves as i have some that have a little hair algae.