“Fossilized” isn’t really a technical term. It literally means becoming a fossil, but that can happen lots of ways, including when the original material just sits around long enough to be considered a fossil. Since the calcite or aragonite exoskeletons coral grows are readily preserved, lots of coral fossils are the original material. So the only difference between “fossilized” and simply dead coral is the age.
The most common fossilized coral, IIRC, is called Petoskey Stone, named for a town in northern lower Michigan, which is a long way from the sea. My best friend picked up a fossilized clam in nowhere South Dakota, though it was paving gravel, so it could have come from many miles away. Much of the US used to be under the sea: you can buy sea salt that comes from southern Utah.
Two things. One, even though rudists were the most common reef-formers during the Cretaceous, reefs of that age with large numbers of scleractinian corals are still fairly common. Most of the so-called rudist reefs still had rugose corals and scleractinians well-represented, and Cretaceous-age reefs where rudists aren’t the majority reef former are fairly common.
Second, the remnants of the Interior Seaway in southeast Texas and up the Mississippi embayment hung on into the Paleogene when the rudists died out. Looking at the state geologic map, it looks like San Antonio is pretty close to the K-Pg boundary, so I would expect to see mostly scleractinians anywhere to the southeast of town.
I’m guessing that someone in the last 50 years dropped it in your field, after they collected it at the beach. I have a piece almost identical to your piece that I picked up in Maui about 5 years ago.
Yeah, what other people have said - looks like brain coral, could be a fossil, could be recent, hard to say if you’re not an expert. But generally, I would not expect to find a fossil just lying in the soil, so I’d vote “recent anthropogenic erratic”. If it were really a coral fossil, I’d expect to find it in a bed of similar rocks with other corals and the like.