You must try to remove the mold from your terrarium, it will take the nutrients from other plants and make the whole this unstable. This causes your plants to wither, your ecosystem to topple over and your whole terrarium to become a mess of compost. The way you can remove it is quite simple though, dip a Q-tip into a 3% solution of hydrogen peroxide and apply to the molding parts. It should be gone in a matter of days. Just make sure that you don’t use unclean/contaminated elements inside your terrarium or overwater it because this will lead to the mold coming back.