Just an update here with the clear pipes. Yup. Green alge on my downspout into the plant bed, and brown alge on my upspout from the pump.

There is some UV resistant coatings that I can get this plastic in... But I'm not altogether sure blocking only UV will prevent alge growth.

It seems the alge building is not the thick 'pond alge', or hair alge. So hoping there will be some benefit. The growth is currently swirling in the water, making a very intricate pretty patterns.