Algae and Guppy Frys?

I have my guppy frys in a different tank, and they are doing fine. However, i can't really afford buying a heater so i use a cap from my other tank and use it as a heater, because the lightbulb gives it heat. The water right now is at the perfect temperature for guppy frys to grow in however, when i feel the side of the tank i feel algae. The clear algae but its slimy. I don't know whether i should give it a water change, or leave it alone. Isn't giving it a change in temperature bad? Is it okay to leave it alone? Or will the algae harm my guppy fry's health?