Yeah I know it's embarrasing. But it doesn't make you a bad fishkeeper. Just makes you a fishkeeper with a problem that needs to be addressed. I imagine the original issue was a bacteria bloom. Possibly mum changed all the filter material at the same time? Or it could have been overfeeding that caused a bacteria bloom. Too much amonia and the bacteria had to work overtime to produce more nitrifying bacteria to catch up with demand.
The 100% water change you did possibly turned it into a double problem. Green water algae blooms a good portion of the time happen in a new tank. Though yours is not a new tank....you did do a 100% water change. But, as I said....seeing what it loks like would be the easiest way to determine what the problem is. Of course seeing in person would be much easier.
You might ask your lfs if they have a diatom filter they would rent to you. Or maybe they know of an aquarium maintenance service that would be willing to help you. I don't know how much that would run...but it's worth asking to find out if it would be a less expensive solution than purchasing your own diatom filter. In any case.....let's see that pic.