Author, A. A. [@username]. (Year, Month Day). Text of caption [Photograph(s)/Video(s)]. Instagram. http://xxxxx
Tyree Library [@tyreelibrary]. (2018, October 12). Check out our spooky display for October! All of the books and movies on display are available for checkout [Photograph]. Instagram. https://www.instagram.com/p/Bo1uWOgAZ6b/
As in-text citation:
(Tyree Library, 2018)
Source: Publication Manual 10.15 (examples 107-108)
This kind of texts is not handled well in a tool like Mendeley and is therefore recommended to do manually.