Humanity has no 'races'.
There are many and diverse ethnic variations - some old some new. The majority of humanity in the universe is from old earth mongolian-chinese stock, but the physical differences are not as easily defined as they were centuries ago due to various social upheavals and intercommunication. There are minor physical differences that have arisen as a result of inbreeding on long isolated subsistence colonies - but on the whole the very concept of race is missing since it proved too difficult to define in any rational way.

If humanity were to encounter any non-human sentient, they would, of course, not be referred to as 'alien races' since that would be a biologically meaningless term. The proper designation is alien species. Unless you can interbreed with them, in which case they would be, by definition, human and not alien.