What is this stuff someone threw in the water?

I’m assuming it’s to attract fish. It’s bright white. One piece is about 8 inches square with rounded corners, by about 2 inches thick. The other piece is shaped sort of like a small dog’s head. Both pieces seem to be the same stuff.

Small bream surround both pieces. It looks like they’re trying to eat it.

The pictures make it look like it’s breaking down in the water, but that’s just glare. When the fish were pecking at it, I didn’t see them pulling pieces off it, or pieces breaking off, so I kind of assume it’s not soft and porous.