Moss needs moisture, shade, acidic soil. It is possible the wood is the issue. The wood or what the wood was treated with maybe leaking the necessary acidic condition to keep the soil PH high.
You can add some lime that will kill the moss but depending on what you have planted it may like the acid soil. Soooo you either may have to learn to like the moss for awhile until the plants are well established and they start to suck up the acid in the soil or plant other plants that like less acid.
