I'm with Jonna, my tropical waterlilies have more like a bulb (kind of potato-shaped) than the long tuber you have.
Dianna, that plant's going to sit and sulk until the water gets warm. You can raise the pot up a bit so it is in warmer water(near the surface) to get that water lily growing sooner. Mine are only about 6in below the water's surface and they do fine. In a pond as small as yours, I hope that's a very small water lily! Keep it in a small-ish pot if you want to keep it from taking over the whole pond.
Plants like water lilies filter the water for you once they are growing. Fish will eat algae, too. It's all about balance, the fish eat algae, their poop fertilizes the other plants that filter the water, and the plants make oxygen for the fish, too. A little algae is normal, and will feed your fish so you don't have to. It takes a while for your pond to get balanced.
