i cannot give any suggestions however i had pretty much the same thing that you described happen to one of my mantids. it started when i fed one of my g.gongylodes a butterfly i caught outside one day. it snapped it up and ate it no problem. however for about a week after it would hang very low and loose, as if almost about to drop from the branches. it looked very weak and would not catch/eat as much as before. also there were multiple lumps of faces building up around the tip of the abdomen. i did not interfere with it, simply kept it as normal. eventually after about a week or two of hanging low and seeming weak, it perked up a little and began to eat agan. the faeces was still a problem though. after a good long while (alot longer than her siblings) she moulted and has been fine since. i have no idea if it was the butterfly or not, only that the weakness and backup began the day after eating this one butterfly. anyway, good luck with your mantis. i would say spraying a little more than usual, not offering some food for a little while and maybe even trying to gently wipe off the backup might help, but thats just off the top of my head. goodluck!
