There are allsorts of algae, its pot luck which your location might favour. You might get any combination of pea green water, filamentous algae (blanketweed) surface coating algaes. Koi will graze on these, silts, sediments, mulm will collect where it can settle.
When conditions go from cool waters to warm, you will find the bubbles of oxygen forming among the algaes will make swathes and gobs of surface coating algaes float about, creating a striking sludge that sometimes floats, sometimes settles after rain bumps the oxygen bubbles out of it
Assuming its the unbalanced pond of water with just fish and rocks in it...
Oh, the herons and predators will have a field day, brightly coloured sushi in plain sight, with no cover or shelter will mean your algae and rocks will be the only thing left to look at
Though, if the algae is prolific and dense, you won't see the fish or the rocks, or the folds...
Regards, andy
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