No worries, our soils are full of fungi and most of these are good for the plants as they help move water and nutrients from soil to roots. Sometimes a decaying root or plant part will host a large fungal colony that grows into the mass you uncovered. I would dig it out and continue with your planting. It could be the start of a mushroom-like puff ball or roots from another member of the mushroom family. Keep growing, Marianne Binetti