Milkfish is associated with Tainan, but Kaohsiung has its own version, too. Instead of serving the meat as is, slivers of meat are wrapped in fish cake processed from the cruder bits of the animal to make sure nothing goes to waste. It's the food of workers and is tasty when fried or with noodles, as it is at this stall.
Milkfish is 虱目魚 (shīmùyú) in Mandarin and most people like to have it in soup with rice noodles (米粉, mǐfěn). This place in an alley only opens for breakfast and lunch, and is always busy. Just see what other customers are having and let the vendor know what you want.