If it moves one end of its body (thinner part) while mostly holding onto something with the wider end, it’s likely a leech. I had a leech scare when getting new plants a while ago. I killed it with salt and isopropyl alcohol, thankfully it never got into my tank, which was thankfully not stocked yet. I recommend you remove it, and kill it. Don’t dispose of it down the drain, put it in a bag in the trash, or safely dispose some other way. you wouldn’t want them getting into your natural local waterways, they can be invasive.
I think you should be fine, i was told they rarely lay eggs in freshwater. (Asian leeches, what I assume yours is)
Addition_Secure 
The shape kind of reminds me of a leech
If it moves one end of its body (thinner part) while mostly holding onto something with the wider end, it’s likely a leech. I had a leech scare when getting new plants a while ago. I killed it with salt and isopropyl alcohol, thankfully it never got into my tank, which was thankfully not stocked yet. I recommend you remove it, and kill it. Don’t dispose of it down the drain, put it in a bag in the trash, or safely dispose some other way. you wouldn’t want them getting into your natural local waterways, they can be invasive.
I think you should be fine, i was told they rarely lay eggs in freshwater. (Asian leeches, what I assume yours is)
rhabdocoela, commonly mistaken for planaria. harmless.
It might be planaria