Five simple ways to rephrase the idea
On rainy days, sloths can look green because tiny algae grow in their fur.
Sloths feel damp when it rains, and algae start to grow on their fur, making them look green.
Algae live in sloth fur during wet weather, so the sloths look green after rain.
Because it rains a lot, green algae grow on sloths’ fur and give them a green shine.
When it drips with rain, algae grow in the sloth’s fur, which makes the fur look green.