Q.What are these bugs in my terrarium?
I have had this terrium for a few months now, made with plants from various nurseries. There are almost hundreds of little white, caterpillar like insects. They have legs leading me to believe they are more caterpillar like than worm like. I need to know what they are as my terrarium is being kept outside which is terrible for it until I know if these are harmful to myself or my cats. In the last photo, a Polka dot plant rotted and died and this seems to be where they are coming from.

These little creatures commonly are referred to as garden centipedes, but they're not centipedes at all. They're garden symphylans, distant cousins of centipedes. You will want to remove this soil for an indoor terrarium. Replace the soil will a good quality potting mix.
https://www.gardeningknowhow.com/plant-problems/pests/insects/garden-symphylan.htm