Scars in a dream usually represent past pain that's healed into memory rather than an open wound -- proof that you got through something difficult. Noticing a new scar can point to resilience you're only now recognizing, while an old one resurfacing may reflect a memory quietly asking to be revisited.
The Moon is in conflict with those who are ruled by Mars, and Mars is in conflict with those ruled by the Moon. - Picatrix (Andalusia, ~1000.AD)