There is somthing weird in my fishtank?

In my fish tank, there are little white bugs kind of things, they're very small, Not even 1mm. they crawls like a snail, no shell. Looks like lice, I have two puffer fish in my 10 gallon tank and some real plants ( not plastics one) I feed dried worms to my fishes. What are those white things and are they harmful? I used to have shrimp in there, but my buffer ate them.