Plecos crack other fish in the face with their tails to tell them to back off- whether being picked on, or over food. They're usually very passive and docile, but, not push overs, and can get kind of territorial. I had to move my common pleco to a different tank with smaller fish as he was starting to pick fights with my goldfish over the food in the tank- The goldfish are pretty large and very pushy about getting food, the fish in the other tank are not. In a tank where he is the biggest fish, he is calm, docile, and happy- in a tank where he feels he has to compete for food, he gets more aggressive. I don't know how old or large yours is, but, the one I have is probably about 7 or 8 (I inherited him with a tank I got recently)... to my understanding, they get more territorial/sassy with age. I'm not entirely sure about your convict's stripes- maybe he colored up more when feeling aggressive and being ready to fight, and faded back down when he perceived the threat to be over and he's now more docile?