Can someone please explain why I had little or no algae before adding the moss?
It's likely a trace of the algae was on the moss, and finding your tank to be an ideal environment, it grew rapidly.
Now the environment has to be changed to something the fish and moss likes, but the algae doesn't.
For starters, put the lights on a timer, no more than 10 hours per day. Make sure the tank isn't exposed to direct sunlight coming in through a window.
With just some moss in there, I would consider you to have a very casual interest in planted tanks; and you probably prefer to solve this in the simplest way possible. If you can, post a picture of your algae, so we can determine what the simplest way is. Otherwise, post a detailed description of the algae, or maybe search for some pictures of aquatic algae to see if you can ID it.