Woke up this morning and found 2 of these white dots between the glass. Fingernail does not catch on inside or outside. Not something that can be cleaned off and I don't remember them being there earlier. I almost want to say it's like a bubble.
I have a similar very tiny transparent bubble in my Marineland tank. Its only visible under certain lighting. The tank has been up & running four years w/ no problems. I just wrote it off as a micro flaw in the glass. Likely no worries.