Welcome to the wonderful world of slime molds! These goopy fungi (Myxomycetes) live on dead organic matter and thrive in moist conditions. You can rake it out and dump it in the trash; especially in the early stages before ithe slime mold spreads its spores. Increasing the air circulation and allowing the soil to dry out will help eliminate the stuff. To help stop the growth of slime mold, just rake any mulch on the soil surface to expose it to air and sunlight. It’s perfectly harmless – just revolting to look at!