A birthmark is something you are born with, so it stands for identity, heritage and what makes you distinct. Old folklore also read it as a trace of a past life.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.