Sorry your question wasn't answered more quickly.
Sometimes that happens,
this site is often more of a chat room than a chicken forum.
Thank you so much for taking the time to respond! yes they have grit. they didn't bother with the oyster shell last night so I think now they know what it is lol
Pics would help.
Gorging on Oyster Shell could in fact be the cause of solid white poops you are seeing.
Something new to ingest can lead to gorging.
Best not to put out OS until they are all laying.
Do they have access to granite digestive grit?
Normally the white is the urates part of a poop,
they don't pee like other animals so the white is their urine.
It might become more apparent as they get older.
18 weeks is about the time that those cute little 'pullet poops' start getting larger.