Think its gone green due to the sunshine that's coming through the window doesn't take much if the weather is clear and being in the house the water will always remain tepid which is ideal for algae growth.
Algae and health wise ... have you seen what wild birds drink from in the wild now that's a total yuck moment, its more of the bacteria that you can't see that will cause health issues when a bird is susceptible which is what the algae start to feed off.
Algae is usually termed as a plant although doesn't have the same things as a normal plant like roots or leaf structure and more often than not single celled structure obviously unless its something like a seaweed ...
en.wikipedia.org
Standardly algae lives from the CO2 intake giving out oxygen and the uptake of nutrients available in the water. I have a feeling your have the dreaded harmful algae bloom (HAB) thought in your head where nutrients are so huge the algae go loopy and mass produces and when it dies and breaks down and decomposes it causes bacteria to go batty and multiply causing a lack of oxygen plus a toxic gas and poisoning the water killing fish etc. Not actually the algae but the bacteria that feed off the rotting algae that cause the issue.
https://en.wikipedia.org/wiki/Harmful_algal_bloom
My African Grey (Henni) had to have her outer beak swabbed earlier this year so we could test for what kind of bacteria are present as she is a mutilator and also a feather plucker, she also had her skin (under her wings) and feet swabbed ….. all I can say is …. I never want Henni to offer me kisses again … what a grotty beak! For some strange reason I thought well she has a clean cage, clean perches, clean toys, I use F10SC she has mist baths etc there can't be that much on her …. WRONG! Some of the results came back with some right scary stuff listed freaked me out but made the vet laugh as he said what do you think you have growing on your skin and clothes? That's the point when I sort of started twitching in the vets office and develop a whole new level to my OCD behaviour I never had before …. calm yourself he said a lot of this stuff is everywhere and if it wasn't you'd be ill as you would have zero immunity to it, its when your already ill that things like that can take hold.
So yes your correct in a way it is best to change water regularly that is being used for such purposes reducing nutrients (supplied by the bathing birdies I hasten to add) so algae doesn't get out of control. Perhaps go as far as not have it standing over a radiator in the winter nor in full sunshine especially when not in use. Then when doing the water change to stop the build up of bacteria a quick swill out on a regular basis with a splash of F10. Or perhaps if its not being used at times leave it to dry out completely.
Who can resist a birdie bathing and they do so like to do it in the sunshine and then they always seem to wanting to sit in the sunshine to dry off and strangely enough they don't always want a bath when we offer it they want it when they decide they want one.
Those budgies are having a wail of a time

BikerGirlCarol is a dedicated and responsible owner so I am positive that she cleans that out on a regular basis, and you must be one as well or you wouldn't have asked the question
