The flexed-biceps emoji is generally a positive emoji meaning 'strong', 'I've got this', 'well done' or 'stay strong'. It is used to congratulate (after a tough workout, exam, life event) or to encourage. There is no hidden or coded meaning in standard youth use. If sent under a friend's gym selfie, it is literal praise; sent under a 'tough day' message, it is moral support. Skin-tone variations exist and carry no extra meaning beyond personal preference. Nothing concerning to look for here.