Nah, it's no big thing... I wouldn'nt use mylar, as it may interact weirdly with your ( I hope) slightly acidic nute- you can little plastic sheets to cover the top of your blocks your local grow shop, if it really bothers you.

I have noticed a correlation between algae and fungus gnats, which aren't too much of a problem for established plants, but are annoying as hell when they fly up your nose.