I liked this story, but at the same time, I would have liked it 100 times more if it was written in a world without The Thing. It is derivative, and eventually falls into fanfic territory, but I think it adds enough insight into the alien's thinking that this story has value above the fanfic nature. The idea of individual vs community, alien vs familiar, evolution and adaptation are all investigated in an interesting(if wordy) way. I only have a passing knowledge of The Thing, and I actually think that may have helped my appreciation of the story, because I was not as caught up with comparing it to the original.
As for the subjectivity of something derivative. There are degrees. Some people can, and do, pull apart every story into neat little categories. TV Tropes wouldn't exist and have such a following if they didn't. but the usefulness of this exercise to me is futile, and often reeks of arrogant elitism to me. Every story can be seen as derivative of some other work, even JR DeRego's Union Dues stories. I feel that a story should be judged on its own worth, regardless of what influenced it. That all said, when a specific work is referenced, it does fall into the fanfic category, as did this one.
On another note, The Thing was released 29 years ago today.