I am sorry to say, but this is no fossil. Corals would show septa in radial symmetry. Stromatolites are very massive due to the fact, that very thin algae layers are laminated by sand layers. Then the next algae layer grows over the sand, followed by another sand layer and so on. This creates quite massive objects. It's also no sponge. They also create quite massive fossils.
What you have is a piece of calcareous sinter, possibly formed in a cave. If you take a closer look, you will see well developed layers of calcium carbonate. These are formed over long time periods by limy freshwater.
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