It may be some kind of insect or disease. I would pull it up and look at the roots, if they are damaged then you should cut all roots away and wash your container and thoroughly rinse plant by rubbing it under faucet then replace new soil and hopefully that works.
@tamsinsbest @Yollymac Upon further inspection it’s entire bottom half roots and stem were rotted. Too much water. Just learned that the pot I was using (large concrete slab) was retaining waaaay too much moisture in bottom half. I luckily was able to save the rest and am no figuring new homes out!