Well, it is hard to tell from that pic. But, I blew it up to about 500% and it is structured like a "honeycomb" coral or something like a Lettuce coral, like
Agaricia humilis. And of either it is just statring to grow maybe from a coral larva. However, there is also honeycomb sponge that is similar.
Honeycomb Sponge ( But I just don't see it and you said it was solid)
Honeycomb Coral
Lettuce
