You see a magic forcefield.
You can see the other side through it.
Notes
A Magic Forcefield is a game square that will teleport your character to a different location when you step on it. Portals cannot be moved, but some may only appear under special circumstances, such as flipping a switch, or placing an item in a specific location. Also, sometimes a portal is present, but it will only teleport you to a special place if you flip a switch or series of switches. Otherwise, it just teleports you to the next square.
Azerus and Versperoth turn into magic forcefields upon death.
Looks the same as a Ship Core and can also appear green.
See also: Mystic Flame