Looks and sounds like CO2.
This is a fast growing weed, so is L aromatica.
As such, they tend to compete with eachother.

For CO2, for nutrients etc.

So if run things lean, you can run into this more, some suspect plant = plant interactions.

I did in the past but later found it was one plant needed more than the other it's rather obvious once you think about, that some plants are better at getting a limited supply under some conditions than others.

So adding a non limiting supply resolves this issue.

Note, if you are CO2 limited, adding more PO4 is not going to resolve the issue.
Likewise, if you are say NO3 limited, adding more CO2 is not going to help either.

So you need to be sure you are non limited for each nutrient, do water changes, clean and keep up on the tank's maintenance well. If you still are having issues, go back through the CO2/nutrients again and reset things again and add a bit more to see each time.

We all have similar issues over time, we all think our CO2 is fine and then have issues only to learn and see later, it was CO2, or we under dosed.

Adding something like ADA Aqua soil + water column ferts gives you more confidence and is easier to dose because the dosing routine compliments the other.

You can simply use a dosing calculator to find what you are adding for N, P, K, Ca, Mg, Traces. Then make sure you do water changes and dose frequently to keep them in the non limiting range. From there, you tweak and really focus on CO2.

Adding CO2 and messing with that should be done when you have a few hours to be around the tank to see how the tank responds.

If the CO2 system is good and well designed, you should see pearling and dramatic growth improvement within 1-2 hours or so and it should stay that way or grow even better as the day progresses.

Adding CO2 mist and good current also can help a great deal.
Rarely is a problem related to low light, this is fairly low light tolerant plant as well.

Regards,
Tom Barr