First pic looks like it’s a fan worm, which may irritate, but won’t sting.
Second pic, it looks like a nudibranch, depending on the type, it would either be harmful or not. Typically, nudis specialize in in a single type of organism, i.e. sponges, algae, zoas etc
Have you noticed it moving around or has it been in the same location?
Try to blast it off with a turkey baster, see if that dislodges it.
Hopefully someone else can chime in with a better perspective