I've noticed this in one corner of my 2.5g bowfront. They're little greenish spots, about the size of the pinhead. Does it look like blue green algae?
Ah...that might be your problem right there. I have a 15 gallon in my bedroom (doesn't everyone have a tank in their bedrooms?) where direct sunlight hits one side, even though the light is filtered through a frosted (bathroom) window. No matter what I do, I can't keep that side clear of algae, and I'm forever cleaning it. If the light is direct on the tank, it'll continue to give you problems.Anyhow, cut down the light on both an extra 30 mins for now, and might even cut it down further as we get into summer, since the room gets some natural light in the morning.