That's a rock sticking out on the left side of this Trident maple. According to Juan Andrade, the maple is kissing the rock (Arce "besando" la roca)

Continuing with the bonsai of Juan Andrade… Juan is one of a whole host of young Western apprentices (Costa Rica in Juan’s case) who are studying or have studied in Japan. This post is from our archives (July, 2017) and the photos and quotes shown here are from Juan’s timeline 

Front view, close up. Here's Juan's caption... "se planto así a propósito desde joven, y el arbol comenzó a "tragarse" la roca. Es un maplecito como de unos 50 años."
And here's my rough translation... It was intentionally grown this way since the tree was young and began to swallow the rock. It's a little Maple about fifty years old 

The other side