They hitched hiked a ride on somthing. Most likly harmless, however unsightly I'm sure. Vacume them out as best as possibile. I asked about the inhabatants because some loches or corys would get rid of the quickly but you don't need to invest in any fish that would become ray food and corys could do some real damage if eaten, so i suggest mechanical removal. Get the vac out and suck up as many as possible with each water change and lighten up on the feeding for a week or two.