yes, it's probably algal growth. when you say up and running, do you mean that the fishes are there already? did you do a complete cycle before you put them in? a lot of factors can contribute to algae in tanks but commonly, it's temperature and lighting. natural light from the sun and high wattage of tank light can greatly increase algal growth. you might want to check
freshwateraquarium for a complete guide. i got through several problems in my tank from it.