Help - sick fish

Anyone who can help, please contact me asap. I have a large (12-14") Koi,
about 3 years old, that has what appears to be Algae (the long variety, as
opposed to slimy film type) down both sides of the fish, towards the ventral
(underbelly). The left eye also appears to be clouded, although not covered
with the same green coating. Through the water and from a distance of about
2 feet, it appears as though the "algae" is growing into and underneath the
scales of the fish.

Thank you for your help,
Ian