Velella velella is the scientific name for a jellyfish species known in Mallorca as barquetes de Sant Pere (Saint Peter's small boats). One of various names in English is little sail; they have a little sail that catches the wind.
On Friday, the beach at Cala Mandia in Manacor was covered with these jellyfish. An "invasion" of this kind isn't uncommon, especially in spring. In previous years they have appeared on various beaches - Camp de Mar in 2022, for example; Cala Molins in Cala Sant Vicenç in 2021. They are blown in by the wind.
This is a species that bears a similarity to the Portuguese man o' war, but only because of the sail. They are relatively harmless.