I think we can say this is a 'cutworm'. It is not actually a worm. 'Cutworm' is a loose term for any one of many moth larvae, usually of the Noctuidiae family, which live in the soil or leaf litter and whose diet includes plant roots. On woody plants they may eat just the soft tissue and leave the wood. But if they eat all-round the stem, that can be enough to kill the sapling tree/shrub. (Other moth/beetle larvae are quite capable of eating wood.) On perennials/annuals they can eat right through the root. Feeding out of sight, the damage may not be noticed until too late - a well-watered plant flagging or keeling over.
They are common, and it is difficult to treat an infection that afflicts plants in the ground (but see below). With pot plants: unpot, tease and wash the soil (and larvae) from the roots, then repot and nuture carefully until re-established, fingers crossed.