Ask Mr. Smarty Plants

Mr. Smarty Plants expertise is in native plants, not native animals.  It is true that freshwater sponges can look like plants because they often have algae growing on them, making them green.  In fact, sponges were thought to be plants until the 19th century.  Sponges, however, aren't plants—they are animals, members of the Kingdom Animalia and Phylum Porifera.  You can read about freshwater sponges in A Field Guide to Aquatic Phenomena from the University of Maine and from the Forest Preserve District of Cook County (Illinois). As to whether they are toxic to dogs are any other animal, not too much is known about their toxicity.  Principles and Methods of Toxicology edited by Andrew Wallace Hayes (5th edition, CRC Press, 2007) says that a couple of freshwater sponges from Australia and Europe can cause skin irritations and even more serious symptoms, including death. You might try to identify your freshwater sponge and then search the internet for any toxicity associated with its particular species.