To dream of crossbones represents a warning of danger or total loss. You or someone else that is communicating how serious it is to avoid something.
Alternatively, a crossbones may reflect you or someone who is projecting a dangerous persona to others.
Example: A man dreamed of seeing a light green crossbones. In waking life he was constantly warning his doctors about how dangerous his medical condition until a breakthrough occurred that gave him hope. The light green crossbones represents the need for dangerous warnings that were being addressed or healed.