This sketch is a "Do Not Touch'' sponge. They're velvety brown, can get as large as a horse, and if you touch, you'll get stung, perhaps severely. Hence the name, which is a direct translation from the Latin, or scientific name, N. nolitangere. I would never call them harmful because the human has to blunder into the sponge. The sponge just sits there, never moving.
I'm wrestling with the idea of selling vs. not selling my little paintings. I'm quite fond of them. In the past, someone bought a nice painting I'd done, and later threw it out in the trash. That hurts.