Trichodina, protozoan parasite. At low levels they don't pose a threat to fish health. However, in large numbers they are extremely irritating and it is likely that some tissue damage will result from the sucking disc that trichodinids use for attachment.
Healthy fish can control the numbers of parasites.
Severe trichodinid infestations are usually associated with overcrowding and poor water quality. Under such conditions these parasites can rapidly multiply. Note the density, with some sitting on top of others. They are easy to recognize in a wet mount. They always remind me of flying saucers, hovering and skimming over the surface of the gill or skin.