"will this get worse?"

Not much.

"does the entire floor need to be ripped up?"

How fussy are you? I'd get an estimate for replacing the floor, cut it in half, and use that as the number you'll settle for.

"how do i convince builder this is a crack?"

Find some pictures of the plywood underlayment.

Thanks for the response! do you think it has to do with improper installation? im not too fussy, in that if it stays just like this, im fine, its not that noticable, more worried about how much worse it will get and if underlying problem that needs to be fixed since its only 2 months old. ill be bothered if it extends across the entire length of the bathroom.

Most likely movement in the flooring system as a whole and no decoupling membrane. Does it run all the way across the room now? How many more tiles are there yet to crack?

Thanks for the response- is a shluter membrane a decoupling membrane? not across the entire room yet, still 6 tiles innthe middle that dont seem affected yet. whats the prognosis?

What was the Schluter membrane put down, it may have been, As Joe mentioned you need to see if the cracks line up with the plywood seams as well, that will be the key.

If i dont have photos, i assume the only way to determine this is to pull up tile? is there anyway to know where plywood seems are without pulling up tile?

This is the only picture i hsve of underneath

Does a main support beam run across the bathroom? And is high in that spot where the crack is occurring, with settling on both sides? When you try to put a tile across a high point ^ you will get cracking, even with an uncoupling membrane. The subfloor flatness needs to be addressed before the uncoupling membrane is installed.

Would it be possible to tell us where in the pic of the underneath the line is appearing? Or perhaps take a pic of the finished room from further back where the line is? It might be helpful to let us know where the orientation of the line is in the room.

" there is a similar line forming on the opposite side of the room at the same location. "

@Minardi, the fact there are 2 places parallel to each other that the same thing is happening leads me to believe it has more to do with the plywood that a main beam.

This is a picture with the 2 areas marked where yhe cracks are. this is on the second floor, so cant see whats happening underneath.

whats the solution? does anything need to be done?

I can't help you with the crack but wanted to say that tile and bathroom are gorgeous!

Hrm.. I could almost accept the line by the doorway might be a scratch instead of a crack if it were by itself. But a matching one across the room right up against the wall where it is less likely to accidentally scratch makes me lean to cracking too.

If they both run in the same direction my thoughts would go to one joist either higher than the others or one deflecting more, similar to what Minardi suggested above. I assumed from your description they were parallel to one another, this look to be along the same line. How old is the house? The fix (if this really bothers you or gets much worse) is removing the floor to figure out why there is movement and fixing that.

House was built in the 60s. do you think it will get mich worse?

And would you recommend i do something about it now or wait to see what happens?

Was there ceramic tile there previously? Was the floor joist system checked for proper deflection with the weight of the plywood and tile? As far as waiting or not, I would want to know what is going on and if the flooring system is safe.

Yes ceramic tile. and no idea if they checked those things

4x4 tolerate can tolerate more movement than 12x24 tile.

I'm assuming this was a gut and start over. Did no one put a level or two on the subfloor before building back?