It's almost definitely a scar, not a birthmark, because of the way Ayua looks at it and says 'I know you don't' (give up). Also, someone with frail bones, wouldn't be doing gymnastics, sports, ballerina splits, fighting with Ayua etc. Too much risk. Her hand got crushed with the full weight of Nika's enormous head landing on it, squashing it. She didn't catching him, but cushioned him to her own pain. That's why it's so charming to see Nika deflect the basketball, and how Sasha is gentle after hurting his leg. They're almost fumbling around each other and it's super cute how awkward they are.