Can you get a better picture? When I tried to zoom in to look at what was coming out it just looks blurry. I can't tell what is going on back there.
When you removed the feces was it stuck? Did you use a bit of force to remove it? If so then yes, you could have prolapsed the anus. Usually you don't have to remove poop even if it is stuck, it may eventually fall off on its own, the mantis could remove it or the next piece of poop will push it out. The only reason you'd need to remove it yourself is if the poop is blocking the ability of the mantis to poop further...but even then you don't just pull it off. First, you would apply warm water for several minutes with a Q-tip to soften it then using the Q-tip very gently try to push it off but don't force it. If it doesn't come off apply more warm water, patience is the key here.
How is your mantis doing now? Does the rear still look the same? Is it able to eat and poop?