If it is a FA type it is probably "spirogyra" as we have it growing well in waters med- high in ortho P and N at temps as low as 42-50F range and often under clear ice.Consider getting a water sample to a lab such as AquaFix www.naturalake.com to see what your nutrient load is. Then perhaps a bacterial program such as using VitaStim Polar may help your situation.I would concentrate on the cause vs the result (symptom)a picture may help also.