Car manufacturers and other industries spend fortunes on branding, app manufacturers by contrast appear to feel obligated to choose a random word and then scream "this is mine now talk to legal la la la I can't hear you".
Ianal but I would (theoretically and against my real lawyer's advice) argue in court that Instagram should have had better foresight than to use an already established word that is taken to mean "short message of indiscriminate content".
Of course the easy way out here is for Insta- i forget, oh yes gram, to pay the littergram bloke's costs for changing the name of his app and then help him to promote the new name. Saying as he is actually doing something a lot more worthwhile than sending shitposts from teen to teen. But why let decency get in the way of POWERSTRIDES THROUGH THE OFFICE LIKE A TRUE TECH-CONQUEROR SPITTING ON THE COMMONERS ... I FEEL SO STRONG NOW, NO-ONE CAN CHALLENGE ME.
I mean, the guy is trying to stop littering, he's not doing anything untoward.