How old is the rabbit in question? Age plays a big factor. I've generally answered for healthy adult buns, but baby and young buns often have a much lower tolerance for fresh foods.
1, Each rabbit will have a difference tolerance. Some can eat as much as they want, some can't. Often spring grass is rich and can upset even the hardest tummy. I guess what applies here applies to all types of food, it needs to be introduced slowly so that you know the gut can tolerate it.
What can potentially happen is that the gut can get upset, which can be shown in things like moist poo, diarrhoea, gas, no poo, etc, so you have to watch your bunny carefully.
2, There can be many reasons why a bunny might get moist poo, but given your previous question it could be related to the grass.
However, generally it can be dietary, medical (such as an illness, parasite), it can also be caused by stress too, and sometimes there may be no cause.
If the issue is dietary then it is worth going back to just grass hay and pushing the hay until the fecals go back to normal, then reintroducing pellets, to start with, and anything else very slowly after the rabbit is settled on pellets and hay. If the poos become not ideal again, then go back a step and eradicate whatever has caused the problem.
You would have to first establish though whether the problem is excess cecals, or if it is the fecals that are moist. Is the bunny doing regular normal poos too?
If the issue is not dietary then the rabbit should go to the vet. In some cases, even if it is dietary the bun should see the vet. If it's not improving with your efforts then ideally the bun needs the vets, and obviosuly if it gets worse, or any other problems are arising, then a vet trip is in order.
