Name of animal found in Boulogne-sur-Mer

[NicholasBio]

Hi,

I'm a biology-student first bachelor and a while ago we went to the coast of northern France (Boulogne-sur-mer). Apart from lots of lots of starfish, me and a friend found a small animal ( 1 - 2 cm length). It stuck itself on some Laminaria sacchari (Sea belt). Now, I asked the assistant the name of this animal. She said she hadn't seen this animal for a long time and that she had forgotten the name. I searched and searched and searched on the internet but I couldn't find any clue. Maybe you could help?

Description:
Length 1 - 2 cm
Dark shell with blueish - greenish neon-like stripes on top (true!).



Imagine the shell something like these sea anemones + 5 lines similar to the leaves of the palm in the picture below.



Really looking forward to your help. Thanks.