Numerous species of snails lay eggs in that pattern including Astrias and Ceriths and from the pics on your Site you have both. It's also possible you have species of snails in your system that you've never seen that are laying those eggs.
Here are a couple of pics of a baby Cerith and a baby Astria both are no larger than 2mm.
And while you may not see babies immediately as they are being preyed upon by your other critters you'll be surprised how many actually survive and are residing in your live rock.
I had snail eggs of the glass about six weels ago. For a while I thought they had all died, ,then suddenly I'm seeing a dozen very small snails. Pretty neat.