Read how to 'do' a Secchi disk.
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92624#Post92624Again - what type of aeration was installed in the one section?
Aerial google view of the loop??
What type of water shed maintains the depth of the loop?
To me it sounds like there is a Cyanobacteria (bluegreen) algae bloom that can depending on the species typically move up and down in a water column. Microscopically analyzing the water will be the only practical way to identify the problematic algae species. I do that type of work along with larger biology departments of larger universities who have algae specialists. Good circulation throughout the loop would have some sort of minimizing affect on the floating cyano surface film. Wind will not cause much if any circulation throughout the narrow 20-30 ft wide loop. Stagnation is very likely one of several causative agents stimulating the algal scum/film.
