Yesterday it was very low tide and while walking in the sea we saw this animal swimming by. I have never seen one before in all these years and I wonder if anybody knows what it is? Some kind of seacentipede or -millipede?

It attaches itself to the coral. It itches if it touches you and may cause a rash but it isn't poisonous. Not sure what its called tho.

I’ve seen these on Koh Chang too. I think it's a type of Bristle Worm. I wouldn’t recommend picking it up as I believe they have hollow barbed bristles filled with an irritating non lethal venom.

Now I know under what name to search on the internet, and indeed it is a type of bristle worm! Thanks for the info!

Wow - never seen one of those before, that is freaky and would keep me out of the sea for a while - where did you say you spotted that?

It definitely is a bristle worm. Many marine aquarium hobbiests consider it a pest. Not sure, but heard that they could grow several feet long. Could vary from species, I think.