The spider mites– yuck! Hate those.
But the millipedes shouldn’t be doing any harm, unless you are sprouting seeds or growing ground bearing fruit like strawberries or squash. I have never had them harm normal, healthy plants. Millipedes are generally beneficial as they like to eat decaying matter like dead leaves and rotten stuff. They help break it down into lovely mulch for you!
Here is a link for you:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7472.html
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