You can break off a piece or take a stem and dip it in a bleach solution. I bleach dipped some of my plants, I didn't have hornwort) to get rid of algae. I suggest using unscented bleach, since I've heard some people having problems with scented bleach. A safer solution would be to soak the whole plant in PP (Potassium Permanganate). It's safer for plants and kills snails, bugs, etc. and supposedely kills snail eggs. I dip all my plants in PP before placing them in my tanks.