This is from felinecrf.org but it doesn't say if it's dangerous or how to stop it, just that if the anaemia's controlled it may stop by itself (??)
"Eating Litter/Licking Concrete
This is known as pica. Many vets are not aware that this can be a symptom of anaemia, but it is surprisingly common, so if a CRF cat suddenly starts eating litter, PCV or HCT levels (anaemia measurement) should always be tested. Thomas was already being treated for anaemia when he began to eat his litter. Sure enough, when we tested him, his anaemia had worsened, so we increased his medication and as soon as his anaemia improved his litter eating stopped".
How is JJ's anaemia and tum? The liquid iron stuff can help a lot, but can cause bloating, stomach pain and constipation.
How's JJ?