Will all great Neptune's ocean wash this blood Clean from my hand? No
Macbeth hallucination of the dagger shows his conscience and consciousness is in disarray because of the act he is about to commit.
While the murder is occurring, outside of the castle there is terrible weather, a symbol of the destruction and chaos that will ensue after Macbeth is "finished" with Duncan.
Macbeth and Lady Macbeth react to the blood on their hands (literally and figuratively) in different ways. Macbeth is overcome with repentance and guilt whereas Lady Macbeth says that this action can be washed away easily.