Dead algae cells are about 5 microns. 160 microns may seem small but a lot of waste is smaller. A mechanical filter doing its job doesn't mean it creates pure water.
The bacteria we care about in bio filters look like:
The black dots are bacteria, the light gray is the biofilm the bacteria creates to live on, the arrow shows the flow of water thru the biofilm channels and the dark gray base is what the biofilm attaches to, like the bio balls.
A single 5 micron bit of decomposed algae cell can fill a biofilm channel. A layer no thicker than the dust on your TV can cover the boifilm completely. That reduces water flow thru the biofilm channels so the bacteria have less access to ammonia, nitrite, O2 and carbon.
Also, waste filled channels allows other kinds of bacteria (bad type) to move in to consume that waste and push out or kill the good bacteria.
This is how bio filters that wash the biofilm 24/7, like Trickle Towers, Bakki Shower and Moving Bed, all out perform static submerged media by 30 times or more
. They greatly increase bacteria access to food and reduce the ability of bad bacteria to kill the good.
Whether or not bio balls need to be cleaned is more of a question of what you need the filter to do. If for example you're measuring ammonia or nitrite then cleaning the media could get the bacteria going again. However, you might have to clean the media daily or weekly depending on the amount of suspended waste your pond has and how water is flowing over the media.
If you're not measuring ammonia or nitrite then there's no reason to clean the media
because it's working good enough, and/or nitrifying bacteria has set up shop other places like inside your pump, inside pipes, etc., where it can stay clean. IMO this is the case in virtually all ponds using a static submerged media filter. That if the filter was removed completely the keeper wouldn't see any increase in ammonia or nitrite. Keepers install a filter, measure no ammonia, and give 100% credit to the filter (and of course themselves). But that's just a lack of understanding the bacteria.
There's been a long time myth that water can't be moved through bio filters too fast
. Often called "dwell time". Very silly myth. If you increase the amount of water going through your filter it will reduce the amount of waste settling on the media and it will need less cleaning. Increase the flow enough so the balls actually move and you'd have a moving bed filter. Bio balls aren't good for moving bed, however they're good in Trickle Tower and Shower filters. So if you can raise the media up out of the water so water flows over the media you should get a much higher ammonia/nitrite conversion rate (like 30x) and you'd never have to clean the media assuming it's kept in the dark.
The issue with pulling bio media out of a filter and washing it,
beyond being a pain, is damage to the existing biofilm. In static submerged media the towers of boifilm can grow very long. When sudden moved around and washed the boifilm can break up and be washed away. In a moving environment like Trickle Tower, Shower, Moving Bed, the biofilm has to grow where it can and to the limits of that environment. So you might get a lot more shorter biofilm, or thicker, instead of fewer but longer thinner biofilm.