Algae spores will always exist in a tank. If your sump is in the dark, you probably won't get much growth in there anyway. Algae needs light to thrive.
to answer your question : algae in the sump won't hurt your tank and it might actually benefit your tank by soaking up excess nutrients. Every so often you're gonna have to scrape the algae out of your sump, if you don't the algae will release those excess nutrients back into the tank as it dies off. But this whole discussion is a mute point here. Planted tanks require nutrients to grow plants, if the algae soaking up all the nutrients there'll be nothing left for the plants. Do you get the catch 22.:hehe: