The move away from peat means that many composts now incorporate a proportion of wood-chip. This is about the only material that is available in sufficient quantity to fill the demand. It can make very good compost, but has the side effect it is also very good at growing toadstools!
These are not harmful and are feeding off the wood, not the living plant. The ones in the picture we think maybe some kind of Marasmius (Fairy Ring Champignon). They are certainly not the dreaded Armillaria mellea. All fungi dislike nitrogenous fertilizer, so if the toadstools offend you, then a sprinkle may just do the trick.