What sushi-like dish consists of various fish in a light sauce?
In general, slices of raw fish or other meats are called sashimi.  These are often served un-sauced, but occasionally they are served brushed with any number of soy-based sauces.

From your description, you could also be describing a Japanese-inspired version of a dish very common in Hawaii called Poke.  Poke is most often made with tuna, but could also be made with other fish or seafood.  The other ingredients included in Poke are highly variable.