What are your water parameters? How high are your nitrates?
You have a young tank and algae cycles can be normal. It takes a while for coralline algae to grow and compete. Before adding any more bioload to your conceivably over-crowded nano tank, I would concentrate on nutrient export... lots of water changes, maybe adding a skimmer somehow. Do you have bio-balls in your filter? Are they cleaned regularily?
I wouldn't add anymore fish in your 12 gal tank... it's sounds like it's already a bit crowded.