In Terminator: Genisys, it's explained that any exposed metal can't travel through time. Only metal that's covered in living tissue (like a terminator) or a fully-naked human. So how can a T-1000 time travel, since it's 100% (liquid) metal?
And the corrupted John Conner himself.– FaizanHussainRabbaniSep 30, 2015 at 9:10
Per my answer here to the same question on Scifi:SE
According to the film's Visual Effects editor, this was left somewhat ambiguous:
Van Ling: That idea (flesh cocoon) was one we had bandied about during preproduction, but it was something that we thought would be too confusing to show visually it would have been like when Brett finds the shed alien skin in Alien. I still think it's the most logical explanation, given we see a flesh "mold" in the teaser trailer already. The other possibilities are that 1) the T-1000 could mimic the field generated by a living organism or 2) Reese really does NOT know tech stuff. Note that several comics and other media later played off the idea of surgically embedding weapons into human carriers and ripping them out of them once they arrived
Originally there was suppose to be a scene showing officer Joe Austin finding the skin. it was something that we thought would be too confusing to show visually it would have been like when Brett finds the shed alien skin in Alien
That said, the film's official novelisation makes it extremely clear what has happened.
They walked underneath them to the place where the two 20-ton plates met and peered within the small gap there. it was only large enough for a single man to step between and there was an indention in each of the plates, an indention in the shape of a man. Connor squinted at something gleaming at the edge of a small round opening positioned where the neck of a man would be in the lower indention. He squatted, feeling his age in his aching knees, and shined his light at what had caught his eye. He carefully extended the tip of his plasma rifle down into the indention, using the barrel of the well used weapon to prod what appeared to be a large drop of liquid mercury. As the barrel of his rifle touched the liquid it flowed onto the barrel of his rifle and seemed to soak in, disappearing.