Looks like a Sword plant and looks more like a deficiency. Trim the affected leaf to promote new growth. Are you putting root tabs under the plant? it could be a potassium deficiency. How new is the tank? the plant in the back left background appears to have a ton of possibly brown algae/ diatoms on its leaf. Do you have a full pic of the tank?
The tank is about a month old now. I haven't used root tabs, but for the first week I had the plant in there I used a liquid Flourish plant supplement (I don't think I had any potassium in it). Also that leaf with the brown algae is part of the same plant. Here are more pictures.