Most likely the blue green algae/BGA hitchhiked in with the plants (i'm guessing they weren't quarantined or pre-treated before usage?)... anyways, this form of photosynthetic bacteria can be quite tough to get rid of once established, so you may need to utilise more aggressive removal methods.
Blackouts seem to work for some people, just leave the aquarium lights off and wrap the entire tank in opaque material for 3-5 days, see if it helps. The plants may suffer abit since they are still not established yet, but the objective is to starve out the BGA.
If blackouts don't work, or the BGA still appear again sometime down the road (due to remnants of the bacteria still lurking in the tank), then you could consider using antibiotics treatment to kill it. As BGA is a bacteria, this is one of the more effective methods to 100% get rid of it, so that it doesn't reappear again. You can try using API Erythromycin for this method. Check some of the BGA removal guides online to find out more options.
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