[fliptuner]

What's underneath? Carport/garage? Can you pry up any of the tiles with a flat screwdriver?

I'm just guessing here but it looks like the grout failed, allowing water to get to the plywood decking. If that's the case, it's likely the mounting screws for the railing aren't fully secure.

If I just wanted it to clean it up, I'd wash the deck thoroughly, give it a few days to dry, replace the railing fasteners, making sure everything was weather sealed, then re-grout and seal the entire deck.

If I wanted to do it properly, I would pull the railing, demo the tiles (which I hate in this application BTW), pull the plywood, inspect and replace any structure, then rebuild from there.