It is a juvenile of Arbacia lixula or Paracentrotus lividus. I suppose it is juvenile Arbacia lixula but I do not know well the difference with juvenile Paracentrotus lividus.
I am not that sure it is a recently settled Arbacia lixula. It could also be Arbaciella elegans. Do you have other fotos or recall if the aculea are flattened?
Arbaciella elegans doesn't live in Mediterranean Sea. There is a study 2011 that explains that all past reports for this specie are incorrect identifications (actually they were juvenile Arbacia lixula). Link