without posting pics, it will be difficult for us to say what's going on - but as the algae grows on your glass, very very small animals that you would not otherwise see will come onto the glass to graze on the algae that is growing there. Alternatively, if you have snails, as was suggested, one may have laid eggs. Most likely, it is not a bad thing - look at them closely and see if they are moving. They are probably really fine and come from your live rock.
Best of Luck!