If those things aren't on your plant and only on the bark, they are bark decomposers ( don't know the scientific name). I have had them on chids I bought and changed the medium and wah-laaa, no more little buggers!
Yes definitely use clean medium .. never repot in the old stuff!
If you reuse the pot, clean it out well -
you can even use a little bleach and let it dry in the sun for a bit.