Hi Rosie and welcome to GoY, we the moss won't be doing your fig any favours and tends to imply that everything is either too wet or the pot is in an area that does not receive enough air circulation and light. I'd remove and add fresh soil.

Agree with Bamboo. For years I've had a fig in a big pot and it has never had moss, unlike so many plants in my garden. When i potted it years ago I put a layer of grit at the bottom but not sure whether this has made a difference.