They may be small pods. Have you seen them move? If so they could be a pod of some kind. They are actually good for the Aquarium, they eat uneaten fish food and detritus. If they don't move it could be a type of fungus or algae. I had the same thing in my Saltwater Aquarium when I 1st started my tank. Did not harm the fish or Inverts. I am pretty sure it is harmless.
As mentioned above looks like pods. I have those bad boys all over the glass and some floating in the water column after I purchased a small wad of chaeto from a reputable seller on eBay.