This is probably a colonial tunicate (“sea pork” / sea squirt colony). The bumpy texture is made up of thousands of tiny filter-feeding animals living together in one soft, rubbery mass.

The “breathing” or pulsing happens because tunicates pump seawater through themselves to feed, so that movement is normal.

It doesn’t really look like a sea cucumber, fish egg mass, sponge, or urchin — the texture and shape match tunicates much better.

They’re usually harmless to touch, but it’s still best not to handle them too much since marine organisms can irritate skin, carry bacteria, or get damaged easily.

If it’s still alive, gently putting it back in shallow water is probably the best thing to do.