Chow Mein: What is the starchy sauce called and how can I cook it?

This is a two part question, both relating to Chow Mein:

When ordering Vegetable Chow Mein from various Chinese restaurants in New York and New Jersey, I find that the dish is prepared with two different sauces depending on the restaurant. One sauce seems like its a typical Chinese "brown sauce," but the other is pretty much uncolored, is starchy, and not oily at all. I'll call this second sauce "Sauce X."

What is the correct name for "Sauce X," and how can I specify it when ordering Chow Mein at a Chinese restaurant?

How can I cook this sauce for use in homemade chow mein?