I would suggest the only way to keep on top of it would be to clean them regularly. A pain in the butt in know but may be the only way

I purchased a home just like that!! It took me days to clean every crevice, screen, glass and other component of every window and door. Gosh it is a relief to get them clean! I feel like I can breathe again…..

mould remover or warm water, detergent and a toothbrush

What is the window frame made from – aluminium or timber???
Do the windows slide sideways or go up and down.
You should clean aluminium once a year. That’s the insructions we were given by an expert. I have also read that in a magazine quite awhile ago. I would try hot water with detergent first. Some chemicals eat into the aluminium. Clean off any dirt and as much mould as you can first. Timber you can also remove the mould by washing with soapy water or anti-bacterial soap in water then rinse off. Make sure you dry it thoroughly. Hopefully you can do it on a warm day so it dries quickly I would wipe it with a clean dry rag or other material to stop the window ledge from retaining water. also the wall around it so it doesn’t drip down onto the frame. If it is timer you may need to remove loose paint and prefer the surface you would when it is initially painted. Use a sealer, not just paint that is supposed to do the job because that method didn’t do it properly the first time. You may have had a lot more rain than normal that has caused the problem.

Vacuum & an old tooth brush for the inside.
Give a wipe over with with some bench spray or Mr Sheen.
Windex on the windows.

For the outside use an old shoe brush & some sugar soap. Give a good scrub before a squirt with the hose.
You can also use dishwash detergent & warm water.

Always brings windows up a treat.
Good luck.

Try vaccuuming out the tracks, and bleach on the mould. It should just wipe off.

A window cleaner should do the job. I spray clean and repeat the process. Make sure the window seals get plenty of sunlight at every opportunity.

outside i would reommend white king to kill the slime/algee/mould. inside i would use the usual clean agent eg mr sheen.