Welcome to the forum. Get a magnet, scrape off the bio film and see if the magnet sticks to it. While some stainless is magnetic, typically in-water stainless steel is non-magnetic (like type 316) because they are more resistant to corrosion than magnetic types of stainless steel.
To me it looks like typical galvanized steel culvert pipe. The ridges give it away. I really, really think you are going to have to drain it so the repair section is out of the water. If the pipe going through the dam is the corrugated galvanized pipe then the best fix is to drain pond down below the pipe, dig it out and replace it with heavy wall plastic or with regular steel (not thin galvanized) pipe.
Look
here for some options on material and supplies.
The pressure of the water in the pond exerts a tremendous pressure on the pipe if you plug it. It would be a dangerous situation if someone was there cutting the pipe and the plug broke free. People have been "pinned" to the pipe due to the water pressure, and if they are underwater, they could drown.
