Maybe. Algae is caused by imbalances in your nutrient levels and light.
You arent cycled, so there are nutrients in the water. You are reporting nitrite. Ammonia, nitrite and nitrate are food sources for algae.
Even when you might consider yourself cycled, as your filter is undersized it might take a little longer than it should to clear the water of ammonia and nitrite. Water should be coming out of your filter clear of these, and with your undersized filtration it could require your water to circulate a few times before water is clear. These small spikes may not show up when you do your testing, but they would be present and feeding algae.
Your filters aren't circulating the water very much, so you are getting dead spots. These are areas where detritus is building up, and potentially nutrients like ammonia can build up in these dead spots too. It's possible that if you carefully took your water parameters in different locations in your aquarium, the bottom compared to the top for instance, you might get find ammonia in one location and not another and this ammonia would be feeding ammonia.
But the most common cause of algae is going to be too much light. Having the aquarium light on too long, or having the aquarium too close to a window.
You only want your aquarium light on for 6 to 8 hours a day, and you want the aquarium as far from a window as you manage. I recently did some lux measurements for another thread. The light close to a window was 20 x higher than it was 4m away from a window. It's incredible how light levels drop off as you move away from a window, but you don't notice it because your eyes are good at adjusting for changing light intensity.