In winter, on a wet day, sometimes you see something coloured like egg yolk up in a tree. It is a mushroom, Witches' Butter (Tremella mesenterica), a yellow gelatinous mass (no, it is not like butter) on a dead branch in an oak tree. In this picture the branch is covered in green lichens.
If it were a plant and not a mushroom you should call it a saprophyte ('phyte' from 'plant'), living on dead organic material, but mushrooms are not plants but part of the kingdom of fungi.The Witches' Butter lives in an indirect way from dead wood, it is a parasite and lives on the mycelium of another mushroom that grows in the dead branch.