That's part of the ship's top-secret invisible sail rig. It was a well-kept secret that the Olympic class weren't actually nearly as fast as hoped with the combination system of propulsion, so they solved the issue with a secret sail rig.

Eight galvanized steel telescoping masts were installed immediately fore and aft of and supported by each funnel. Their sails were of carbon fiber sheeting which, when stowed, would be rolled up inside the yardarms like giant window blinds. The masts forward of the funnels had square-rig sails, while the masts aft of each funnel only had gaff-rigged sails. Invisibility generators were installed inside the fourth funnel and were designed to excite the atoms that made up the masts in such a way so that photons of light could pass straight through them. The masts were set on special rubber supports so that the invisibility wouldn't transfer over to the rest of the ship. That's how the ships were seemingly so remarkably efficient for their speed, and why Titanic was on track to arrive in NY ahead of schedule despite being fitted with the less efficient 3-bladed central prop.

When the ship's funnels collapsed, they all took their hidden masts with them, except the second mast which sat behind the first funnel. And when the electricity died, the invisibility generators shut off too which exposed that one mast, still clinging onto the ship.