It looks like BBA (black beard algae). It's a tough call on how to eliminate it best. You should provide us with a lot more tank details (tank size, lighting, filtration, any CO2 supplementation, fertilizer dosing routine, etc) so that we can better assess the situation.
I cant find excel either...
i have removed all the infected leaves in my tank,only on the plant in the first picture i have left it as it is...
i have direct dose the plant with H202 2 days ago,(2ml/gal)..i algae is still here on the plant in the first picture ,but the colour of the strands have become a bit brown/grey
Al i have lost ALLLL the leaves of my hornwort.....:/ :/
Oh no! Everybody remembers to post that vals and Eleodea are sensitive to Excel but hornwort is a bit touchy. I wonder if removing it before treating the tank and putting it back after the bubbles are gone would have saved it?
A change in color is what you are looking for. It was killed. The trick is keeping it gone by the small changes suggested - reducing the lighting period and trying to keep CO2 more consistent. I read that using a couple bottles on an alternating replacement schedule works best for DIY CO2.