That's an infantry unit on a convoy in the same location as a destroyer.

When land units move onto sea tiles, they board convoys. When you have that convoy at the same location as any other naval vessel, they'll merge into one 'army' just like units do on land. The representative symbol you see is only showing you the contents of that convoy.

The 'symbol' is representative of the stronger of the units in that stack. If you split the convoy from the stack, it'll have its own symbol.

Hey... you have the same first part of your name as me. Plus you also have 4 numbers after it.

Units which are in the same location almost always form a group. This group takes on one image as the dominant image which represents all of the units present.

This group will move as a single cohesive group within the limitations of the slowest unit and terrain restrictions of all the units.

To separate the group use the split command and give the selected parts an order that differs from the current group.

This is the standard function of the game.

Having a group of units allows them to move at a constant speed. A larger group of varied units is more resilient to an attack, particularly in this case having a destroyer or two as part of your naval convoy group will defend against enemy submarines better.

We have 3 killers now...