Yup, with that much light over a 15g and no co2 injection you will have algae. I recommend injecting co2, or putting about 25-30 watts of light over the tank instead of 55.
For now, manually remove all algae if possible, and any algae infested plants. Then, STUFF hte tank full of plants most importantly some fast-growing floating plants, like hornwort. They will block some light and soak up nutrients.
Great looking fish by the way!! is your water naturally that basic, or do you buffer?