Here is a clear, kid-friendly grammar correction:
- Original: Sloths have green fur on rainy days because of algae.
- Corrected: Sloths have green fur on rainy days because of the algae.
Why we add the before algae here: In this sentence, we are talking about a specific type of algae that makes the fur green, so we use the (the algae).
Tip for young readers: If you mean any algae, you can say, Sloths have green fur on rainy days because algae live on them.