Jace Omg!! I know! In the mortal instruments the inquisitor recognizes jace, as a herondale, i guess because he has a star shaped birthmark( cuz Tessas necklace burned will herondale) on him as did jace father and the herondales before him, that's why it was mentioned!
Rita Margarita The star got "burned" on his shoulder by Tessa's necklace. It touched him when they slept together in CP2.
Arianna The star on Will's shoulder is actually Tessa's clockwork angel mark on him. That's why Jace has the birthmark.
This answer contains spoilers… (view spoiler)[Will got the star when tessa's clockwork angel burned slightly on his skin when they slept together in Clockwork Princess. The clockwork angel contains a part of the angel Ithuriel's soul...After that,all the descendants of the Herondales have that scar which would explain jace's scar:) (hide spoiler)]
funny watermelons After Will found Tessa in Cadair Idris, and revealed to her that Jem had died, they slept together after hours of crying and softly conversing. While they slept, Tessa's clockwork angel fell against Will's bare shoulder, and left a mark.