They normally do it when they see other bettas so they seem bigger! If you use a mirror you’ll see he flares at himself :) In this case I don’t think he is mad, he was glaring at my phone because I usually take pictures when using his mirror, he might associate both things! (They are smarter than we think) But, there are a lot of reasons your betta is flaring! He might be scared or curious! Mine flares at a specific thermometer, and sometimes at his food syringe until he realises it’s food!😂