Certified GKH Gardening Expert
It sounds like you have a slime mold, which is feeding of decaying parts of the plant. Clean it off and treat the plant with fungicide. Since it feeds off decaying plant matter, this means the tree fern, at the very least, has damage. If it is still alive, it may recover, but will need a careful eye on it to look for pests and disease that are prone to attack sick plants.