The only altitude information TCAS ever displays is that relative to the other traffic. The traffic symbol can have a 4 character tag; A plus/minus sign and 2 digits for altitude difference in hundreds of feet and an up or down arrow if it is climbing or descending more than 500 feet per minute. Once again, like the transponder, this is all based on a hardwired standard pressure altitude setting, so the pilots cannot stuff it up with an incorrect altimeter setting.
Almost exactly what I would have said, I'd just like to clarify the plus/minus bit, you would see in this case (if the intruder were 100 feet above you descending at more than 500 fpm.
If an RA, a red square above which in red would be, +01 with a downward pointing arrow. This can be switched to display flight levels, as per DD's explanation.