The Twitter account is not confirmed to be official, but James Gunn did delete a tweet stating it was a fake (which probably means the account is related to promoting the movie).
Taking a look at the image below, obviously the first guess might be some sort of star. Perhaps on Captain America's shield? However, the star on Cap's shield is surrounded by blue, not red. There also looks to be a line of blue going through the bottom of the white part of whatever the image is. Maybe the paint gets messed up in a battle with Iron Man? Or perhaps this is an image of a patch or part of someone's costume?
The Russo Brother Twitter did seemingly tease the Spider-Man costume, maybe this is related? Any ideas?
"Captain America: Civil War" has a May 6, 2016 release date directed by Anthony and Joe Russo starring Chris Evans, Robert Downey Jr., Scarlett Johansson, Jeremy Renner, Elizabeth Olsen, Sebastian Stan, Anthony Mackie, Frank Grillo, Paul Bettany, Don Cheadle, Emily VanCamp, William Hurt, Martin Freeman and Chadwick Boseman as Black Panther.
Captain America: Civil War picks up where Avengers: Age of Ultron left off, as Steve Rogers leads the new team of Avengers in their continued efforts to safeguard humanity. After another international incident involving the Avengers results in collateral damage, political pressure mounts to install a system of accountability and a governing body to determine when to enlist the services of the team. The new status quo fractures the Avengers while they try to protect the world from a new and nefarious villain.