General Water Softener Question

I'm not very water softener savvy, just learning. There is a 1" flexible tube that runs from the softener unit, above the basement ceiling, over to the other side of the basement where is connects to the sewer line. The section of tubing closest to the sofener has green algae growing in it. What exactly is this line for? I assume it is some type of vent line? Thanks

General Water Softener Question