This little fly (size estimated) was hanging out on the surface of a fresh slime mold (Fuligo septica) that emerged in a mulch-filled planter. It was waving its wings intermittently, and I saw it back down into a little crevice in the slime mold surface and, apparently, oviposit. (I believe it drank fluids from the surface as well.)

In this photo, the fly is crawling along the surface and waving its wings.