either leach or planarian, not sure but suspect the latter. Doesn't matter too much either, but that the leach is vaguely more predatory to small stuff. They're likely thriving on the leftovers from notoriously-messy pufferfish
Some/most fish will eat them. Think about loaches or cory school (albino?) to both eat the beasties & compete for food-bits to starve them out, as should your Macrobrachium(?) prawn & guppies & even the BNPs.
In your stock they're unlikely to be a problem, only aesthetic.
Unrelated: What puffer sp. is that? How is the shrimp surviving??? - impressive