A person might dream of decay as a sign of mental illness that leads to death.
Dreams could portend the deterioration or decline in a particular domain of life, one’s health, one’s abilities; literally, dreaming about something decaying means that an associated element of life is gradually eroding away.
It can also be seen as an indication of your mental state. A dream about something rotting may represent aged thoughts – putrefaction stresses the need for renewed creativity, ideas and different perspectives.
Death appears in many of our dreams, not least with explicit reference to decay and such deaths usually suggest the dreamer believes their mental or physical “health” is failing.