If you offer an application, website or product that is compatible with or related to the Instagram service, such as an app or website that uses the Instagram APIs, you may fairly and accurately describe how your application, website or product relates to Instagram. But you may not use the Instagram trademarks in your own brand, or suggest that Instagram sponsors or endorses you or your products.
Don’t:
Use “Instagram,” “IG”, “Insta” or “Gram” in your app or product name or domain name
Use the Camera Logos, or the Instagram name or logo, in your brand identity, such as your app icon, logo or product design
Use the Instagram stylized font in your logo or product or marketing materials
I found the specific language in their Branding "Guidelines" that forbid the use of "Insta" and "Gram" in app or domain name.
Here's the link: https://help.instagram.com/304689166306603/
Relevant Section:
Naming Applications and Products
If you offer an application, website or product that is compatible with or related to the Instagram service, such as an app or website that uses the Instagram APIs, you may fairly and accurately describe how your application, website or product relates to Instagram. But you may not use the Instagram trademarks in your own brand, or suggest that Instagram sponsors or endorses you or your products.
Don’t:
Use “Instagram,” “IG”, “Insta” or “Gram” in your app or product name or domain name
Use the Camera Logos, or the Instagram name or logo, in your brand identity, such as your app icon, logo or product design
Use the Instagram stylized font in your logo or product or marketing materials