I suspect white fuzz algae, but can't be 100% sure based on the photo. Like most algae reducing your photo-period and/or light intensity is a good start like 4-5 hours a day. Also upping your maintenance really helps e.g. removing dead or dying growth, increasing the amount or frequency of water changes temporarily, deep cleaning your filter, substrate vacuuming, etc... I've heard of spot treating with Excel but I've personally have not tried it. If its isolated to that plant and the top part looks healthy and algae free, trim and replant removing the lower older portion is another option.