I've raised this topic recently to our forum buddies, and unfortunately, never actually got an informed response from anyone as to what they were. Like them, I'm not a worm specialist, so don't blame them.
I think you may have got what I recently had. I don't think they're Planarians, because Planarians, so I've read, are far longer than the 1mm you/I had described.
I still haven't found an informative article in any of the websites I've seen to identify them. Perhaps, there's a worm specialist out there in this chat forum?
I think they're a part of your tank fauna, so they could be good for your tank. Who knows? Personally, I didn't take the risk and decided to undertake a big clear out of everything, including a thorough clean of my gravel substrate.
You may go down this route yourself, up to you. Other options suggested are:
1) turning up the heat to >30c
2) decreasing the amount of fish feed
3) medication (wouldn't recommend this)
4) regular maintenance of your water quality
Your choice, and good luck.
