Her tank is pretty small. What is the temperature? What kind of filtration do you have? How big is she? She may not technically be a hatchling anymore but the information on my page at
http://www.fishpondinfo.com/hatchling.htm should give you an idea of what her setup should be like, foods to try, etc. Turtles can go a long time without food but it's not normal for such a young turtle to do that unless she's too cold or is ill. What did you add to kill the algae? I suggest getting her a large home, trying some live foods like earthworms, etc. Do you have both incandescent and fluorescent lighting? If she doesn't improve, take her to a vet to see if he/she can help.