You might consider posting this in the Butterfly Forum but what your describing sounds pretty lethal to me.

Hello Julie don't worry about the caterpillar, these symptoms sound like bruising on the inside, it's good that it's moving. It needs to stay on a plant and rest, try to restrain from making much contact with it so it can heal up

Hi all, thanks for your helpful comments. I know it sounds silly but I felt bad for it. I usually don't interfere with nature but it was going to be eaten. It was lovely and walking around on my hand for ages. Tauryn, I left it on a dock leaf as this is what the Ermine moth in my region eat. I checked on it a couple of times and it was moving. It fell off the dock onto the soil so I left it instead of touching it again. ( I hid it slightly so birds wouldn't get it) Anyway, I went out to check on it today and it's gone so I am hoping it healed up and went to pupate !

HU....I believe that you posted this response in the wrong thread, lol.

Is it ok to pull out the gook?

I need help! I have a pet caterpillars and they were dancing and then something fell down on one of them! its eye is injured and black goo is coming out! Its name is Crystalline, she is still moving a lot. Edit: I can't find her.

Same

sounds like pesticide poisoning, but i'm no expert

It seems like anal prolapse. It's

My catapiller has it's but part sticking out of it's body but it's still attached and it's Alive it's outside of it's body but it's still moving I dropped it by accident but it appears to be Alive and it's eating leaves does it go back inside when it heals or should I leave it alone until it turns into a chrysalis?

It's but part is sticking out the part where it poops from

Just keep it separate and monitor it. If you have reddit i would ask the monarch butterfly group for some better advice