[magic452]

The gray icon is an unidentified ship sunk by some means other than you.
Heavy seas or enemy action are some of the causes.

The red icons are identified IJN ships sunk by some means, heavy seas or enemy action. In most cases your torpedos or deck gun but also could be Allied ships or planes that caused the sinking.

Blue icons are allied ships sunk.

Sometimes if you sink a ship and it doesn't sink right away you may get a gray icon. Other times you get no icon at all. You will still get credit for the sinking.

Allied and IJN forces do interact, just go on one of the quick missions and watch all the action. Crank up the sound to get the full effect.
You can have all kinds of different fun in this game.

I recently loaded up Rockin Robbins slightly sub nuclear torpedos and wanted to see what effect I could have on the Battle of Sumar.
I fired 4 fish and before any of them hit US planes and ships sank all four targets.
I dove deep to avoid a DD and by the time I could come to periscope depth every major IJN ship had been sunk. There were only a couple of DDs left.

If you haven't heard of sub nuclear torpedos, just one will decimate the Yamato. Kind of fun for a change of pace.

Welcome to the Forum, good luck and good hunting.
Magic