Yes, it is a flatworm. They can come on the LR. Nothing bad about them, they just may not look all that good. You can get a velvet nudibranch, they eat flatworms, so you can get rid of them. The only problem with that is the nudibranch will starve after it ate all the flatworms.
WOW! Good timing guys. I was digging through som LR rubble I got at a LFS to spread the pieces around a bit and found lotsa critters. Several of the macro stars, which seem to be multiplying well, a tiny juvenile mantis shrimp (1/8 of an inch) which was going to go into my 29 by his lonesome untill my lion potted him and on the very last rock was a solitary flatworm. If it wasnt for this thread, Id have no clue what it was. Anybody know if these are asexual and what are the odds of the takn having more? And what exactly do they do? Thanks.
Reading here, it seems they do no harm unless they reproduce to epidemic quantities. They do reproduce asexually. Their are some treatments for the tank that can get rid of them if you want. If you see any I would think there are many more that you don't see. They seems to be one of those "critters" that live in your tank.
it is definetely a flatworm, and I suggest you kill them all quick. I had a few at one time, and now my tank is infested with them. they reproduce like crazy, and will cover any zoos causing them to close. They also let out bad toxins when they die. You dont have to take my word for it, just post in the reef section.