She's posing in the sand outside of a what appears to be a beach house, and is holding a can of Diet Coke.
"Thank god she finally got over her shyness," Willis' full comment reads.
Getting over her "shyness" isn't the only big milestone Willis has had to celebrate in the past year, as last July she opened up about her struggles with sobriety and other health issues.
"3 years ago I was a malnourished string bean with aches that echoed throughout my soul. However the internal cries to tend my most blistered and deep wounds repeatedly fell on deaf ears," she wrote in a post on Instagram.
"I did not value myself, my life or my body and as such I was constantly punishing for not being enough. Self annihilation fueled with medicating left me a shell, and the world on mute," Willis continued.
"I was hoisted from my hole, (one so deep I was certain we were nearing the Earths magma core) on the backs of powerful human beings that I will forever be indebted too, and on that day my life was gifted back to me. I love the girl in this picture, I cry for her and I mourn her lost years," she added.
"She is inside of me always and I must never let her slip too far. I don't push any agenda, I can only speak for my path and staying sober has been far and beyond the most important thing I've done in my wee 23 years," Willis concluded.
Willis is the youngest of Moore's children and has appeared alongside her parents in a small-handful of films.
When she was a baby, Willis appeared with Moore in 1995s The Scarlet Letter, playing the role of Peral, the baby born out of wedlock to Hester Prynne and Rev. Arthur Dimmesdale.
In 2001, when she was a little older, she had a small part in the film Bandits, which starred her father Bruce, as well as Billy Bob Thornton and Cate Blanchett.
Willis' final film credit came just a few years later when she appeared with her father again in The Whole Ten Yards.
While she hasn't done much acting as of late, she did contribute vocals to the song "Tamale" by rapper Tyler, The Creator back in 2013.