There's one thing I've not understood, what exactly I the "birthmark"? I couldn't find much information on it In the game besides the MC mentioning it, far first I thought Asterion just got some dirt on his head because he was face down on the ground and his fur was white when it first showed up xD
Pretty sure the game does not conclusively explain what it is yet. The most we have at this time is theories on the birthmark. All pretty much agree on its great significance though.
Just a heads up: thanks to the forum setup we have right here, it's easy to look up topics on like this. There's a search function in itch.io that can pull up mentions of a term and such. Give it a whirl!