The critters are millipedes. They don't damage plants. They eat dead organic matter.

Don't know the source the the "poops."

are these indoor plants that were outdoors for summer???

i usually treat all my summer outdoor plants .. outdoor in early fall... with a systemic insecticide ... so that when i bring them indoors ... they are bug free ... MI winters are to long.. to be dealing with vermin in the house ...

many peeps dont like to use said products indoors in winter ... so your only option is bare rooting and repotting ...

i would get a high quality potting media .. not MG ... i would dampen it properly.. and then i would sterilize it ...i learned not to rely on whatever the bag claims ... do it again ...

http://tipnut.com/sterilize-soil/

since you have it out of the pot.. it might be a good time to get a slightly bigger pot ... if not.. wash out the old pot with 10% bleach ...

repot.. and be rid of your interlopers ... harmless or not.. i dont want bugs in my house ...

you might get a lot of info.. in the houseplant forum ...and if outdoors.. the container forum ...

ken

ps: if your grow these outdoors ... in your zone.. ignore them ... there arent an issue ...