NTR does not imply there's a relationship. You can have NTR without cheating or relationships.
NTR implies that a person's love interest or loved one is stolen away.
A separate tag was created to take care of most of the situations I'm talking about. It's called BSS. It works, but it when it comes down to it BSS is really just a niche form of NTR.
For an example, a common story with this theme is between childhood friends where one of them wants to be together and the other gets into a relationship. It's NTR even though the childhood friends were never together. The same with a single/widowed mother who's taking care of her kid, but all her love and attention is stolen away when another guy corrupts her. That's also ntr.
On F95 NTR should only be tagged if the MC feels jealousy over his/her love interest being stolen away. I kinda doubt Grolokk has feelings like that so it probably shouldn't be tagged. I haven't played it to 100% completion though.
https://f95zone.to/threads/tags-rules-and-list-updated-2021-05-02.10394/