So, velella vellella do have stinging cells like most Jellyfish. However, not seriously dangerous usually to humans

 But, these stinging cells on hands or barefeet might cause irritations but not welts usually like some jellyfish will.

So, mostly don't grab the blue ones with your hands so you don't irritate your hands or face from touching your face or eyes or mouth. But, you might feel irritation on your bare feet if you step on the blue ones that just reached the shore before they dry out to looking more like clear pieces of plastic within a few days after bleaching out on the beach for awhile in the sun.

They are all over California beaches right now so just be aware of what they are and that they do have stinging cells but are not usually harmful to people unless those people are complete idiots the way that they are dealing with them on their bare feet hands or faces.