I just focus on their expression to differentiate them, Bae Sejin usually have a glare or a little furrow in his eyebrows while Moondae is always neutral or listless lmao.
Two other things is that Moondae's bangs look more upside down triangle like and his hair is messy with hair sticking out here and there. Sejin A is more clean and somewhat straighter.
I keep mixing up Moondae and Sejin A after Moondae's hairstyle and hair colour change! (╯°Д °)╯╧╧
Atleast change the styling up a little omfg!!!