I don't think its that uncommon for Discus to nibble on plants. A lot of my Discus nibbled on my plants when I had them, especially my BD. She even ate plants that supposedly fish don't like, I think it was the Java Fern. Now, when I feed my BNs algae wafers, the Discus are the first ones to the wafers and the BNs have to hope they can get a bite. Possibly your BD may not like the food you're feeding and is eating plant matter instead. He was eating other foods before, wasn't he, and then stopped? Did you change the food you're feeding?