From what you've said, can't help with a diagnosis, sorry. But in any case, the usual response to a bacterial bloom or an outbreak of algae should make it less ugly to look at.
Do a series of large PCWs over a one and a half - two week period. Do 70% on day 1, 30% day 2, 70 day 3 and that *should* remove the bulk of it. Then 30% every second day for the next week or so.
Can you take a photo by chance?
Sharon : Melbourne, Australia
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4x4x2 FW planted | 15g FW planted | 3.5g FW planted pico