Horsehair green FA can be a BIG problem as you are seeing. Technically Pithophora to my knowledge does not grow in MI. Mostly only algae geeks know this. What you have is very probably Cladophora a world-wide very common horsehair-like algae and can have a lot of morphological variation thus making it sometimes hard to identify. Pithophora is primarily a subtopical or tropical species similar to coppernose bluegill. The main technical difference between Pithophora and Cladophora is Cladaphora rarely produces resting spores and Pithophora commonly has spores with branching that occurs close to or near the crosswalls. Both start growth attached to the bottom or surfaces, get long and stringy and eventually float to the surface in big masses. Anyway both are very problematic algae species. Your algae problem still exists regardless of what its technical name is.
Best natural controls are tilapia and possibly white amur aka grass carp, however neither are legal in MI. Your other best natural approach would be to introduce competitive plants to cover the bottom and out compete the FA with a bottom covered by rooted plants competing for both nutrients and space.
Another option to control this is to try products by Aquafix from a company associated with Univ Wisconsin Madison WI.. They have a recent newsletter that described products that controlled problematic Cladophora and Pithophora. Search for copies of their Winter 2014 and Spring 2014 newsletters.
www.naturalake.com
