Any "white-ish" areas like that should be a focus of concern. Looks like it could be a bacterial infection. At a minimum, I'd consider giving the puffer a 60-90 minute bath in Ruby Reef Rally. It contains both antiseptic and antibiotic qualities. If no improvement, treatment in a QT with NFG (nitrofuracine green powder) or the "trifecta" of Kanaplex + Furan2 + Metroplex.