1 - try to close world editor and open it again;
2 - look at the Object Manager and check all doodads you have placed on your map;
3 - it might be a structure that was removed, but left it's shadow after it, follow step 1;
4 - run the map in War3 and check if the shadow is still there;
5 - check if you haven't made objects with 0.01 scale size (objects with no model are griddy squares).
If no steps worked, try to place something else on that shadow and none will notice it
Ideas for your Real Life on Board:
-Casino: buy a check that costs gold. Step on a circle and all checks will be removed from your inventory. Stay where you are. There are three holes in front of you. Watch for Differently colored footmen running out from holes in front of you. There are three possible colors of footmen and if all three footmen came from the hole are of the same color you get your checks doubled.
-Job: carry the piano from the ships down part to the casino room.
-If you don't eat, you die of stravtation.
-Your mana always decreases. When it drops to 0.00 you suddenly fall asleep until your mana fully restores. You lose half of your total health after sleeping on the ground. You may restore mana by drinking (minor restore) (drinks provide much else effects). Yoyu can avoid losing health from sleeping by using your bed at your room instead. Hit ESC at any time to awake. Mana restores very fast while using bed. You may be awakened if someone hits you.