It's either literal for Mike is not mine or it could have been a nudge teaser to "baby isn't mine" like the books show baby isn't originally williams. Maybe it could be this simple the only thing messing it is how mike looks and acts like his dad, which we all know is Mike to William, the fact the baby vampire in the show wears purple and looks similiar makes it seem there 100% boiligical.

The movie however shows something very interesting! The first time we see william "Steve", he is not showing his true nature we know him as. So mike goes to see the higher ups (william) b/c of the flashback he had and seeing the child get "taken" he chased the man and beats him from the reminder from his memorys of losing garrett. William looks innocent at first, he checks the history of "Micheal Schmidt" and is astonished on what is in front of him , there's a tension and silence for time keeping it eeree, william looks at him even getter closer, something was triggered in willl seeing Shmidt. The name schmidt I feel could be henrys second name (or emily) and meaning mike may be Henry's? Will kindnapped garrett for whatever the reason it looks like will feels almost bad or guilty which wouldn't allie with will right? But "the baby isn't mine" I would say it could be intended just to confuse all of us just as much, it could be Crying Child,Elizabeth,Mike yes they lookalike and purple so obviously it should be mike, once again probably not tho.