How do I stop algae growing on my soil?

You are over watering. Let the soil dry out. Algae wont harm the plant. It is eating very little nutrients. When the canopy covers the soil it will also block the light and the algae will go away. You can also cover the top of the soil so it blocks the light if you want. Algae needs light.

Scrape it off if its bad. Your probably watering to frequent.

Too much water. I am big on planting in a forever pot and never transplanting but it does require proper moisture control. You do want to water well beyond the perimeter of the plant to encourage the roots to grow out as far as they can. But, without roots at the edge of the pot consuming water it is easy for those areas to be over watered. I stick my finger in the soil and water when it is dry down between my first and second knuckle it is OK to water. If the soil surface is damp or damp less than one knuckle deep do not water. With your plant you might have to water around the base of the plant every other day but could do the perimeter of the pot every 4 or 6 days.