I've been getting a lot of those. As far as I can tell, anywhere that I've put coffee grounds, and then have added sulphate of potash, they spring up like crazy. They don't seem to cause any issues though for veges. I have found with a few of them, they fluoress if you put vinegar on them (similar to how some stinkhorns do - though I'd take these slimes over stinkhorns anyday). I will take a photo next time I get one. One day, I had one in my coffee plant pot (which is basically 90% coffee grounds that they are growing in), and overnight, it took up the entire plant surface, and climbed down the side of the pot to the deck.