PePa

It doesn't depend on acting like a farang. Thai people have a definite category in their minds of white people that are a distinct group in their perception and experience. They will just refer to a white person as a farang because that makes sense to them. Only if they both know you and know your name they can refer to you using your name. It's not meant offensive, and it's unrelated to how well you are integrated into Thai culture, language and society.