Kerang masak is blood cockles cooked in either a spicy sambal or a soy sauce (kicap) gravy. The cockles themselves are genuinely lean, around 105 calories for 130g of shelled meat, with decent protein for a shellfish. The problem is the sambal. That chili paste base with belacan, shallots, palm sugar, and a generous pour of cooking oil adds roughly 200 calories on its own. So you are looking at about 300 calories for the whole thing, which is not bad as a lauk to go with rice, but you want to be honest with yourself about how much gravy you are scooping up. To be clear, these numbers are a ballpark estimate because solid nutrition data for this specific dish is thin. Take the figures as a useful guide, not gospel.
What actually changes the maths here is the sauce version and how heavy the cook's hand is with oil. Go for kerang masak kicap instead of sambal and you save roughly 70 calories per serving, dropping to around 230. The sambal petai version adds stink beans and bumps it up to about 330. If you are at a nasi campur stall, use a slotted spoon to grab mostly cockles and leave some of the oil-heavy gravy behind. You can easily cut 50 to 80 calories that way. Kerang is also a solid protein choice at 20g per serving, so pairing it with plain rice and a vegetable is genuinely one of the smarter hawker plate combinations you can put together.