Algae build up from pond is clogging sewer drain
That looks like the type of algae that is floating IN the water (as opposed to ON it). That means it might be controllable via aeration, either a redesign to add more falling water to get more oxygen into it, (i.e. a small dam and waterfall) or by adding a small pond pump and sprayer.
Secondary to keeping it under control is to take a look at whether you are causing it to over bloom in the first place. Looking at your yard, you might be over feeding your lawn and plants, so the nutrient runoff is getting into that stream and feeding the algae too.