Water mites is a group of about 5000 species of mites. Scientific name of them is Hydrachnidia or Hydrachnidia or Hydracarina or Hydrachnellae. They are usually very small and this is one of them. I found it in my freshwater aquarium on Helisoma anceps. It is so small that I had to use lens to verify, that the small dot on the shell is a water mite. 
The water mite is in the centre of the photo. It is on the mantle edge near the aperture. Detail cropped from the previous photo: 
You can compare the size of the water mite with two Ferrissia fragilis snails on the top of the shell of Helisoma anceps. Both of these freshwater snail species are of North American origin and both belong to the same family Planorbidae. Ferrisia fragilis can reach 3.2 mm maximum shell length, but I think my snails are a bit smaller. The water mite is about 0.5 mm or smaller. It was actively moving on the snail. The dot in the centre of the photo is the water mite: 
I do not know the origin of the water mite. I found only one in my aquarium meantime.
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gastropods 
Six Years Later… I know. I’m searching the net but I can’t find if water mites are harmful to snails (mine are nerites). Were your snails ok with these neighbors or their larvaes end parasiting the snails? Thank you ❤
Thank you for your comment. There was only a sole mite in my aquarium. So it was not harmful in my case. – I hope it is safe if there are not plenty of them.
Thank you very much. Seems I’m having a small population explosion, caused, I think, for the abundance of algae. I hope that mites ends regulating itself, but I’m afraid for my nerites because I had see mites in the “snail’s house” (Sorry, I’m from Spain and I don’t know the English term :P). Thank you very much for your kind answer and congrats for your webpage, your photos are also great.
Greets, Eva.