Slash -- No Shirt, No Shoes ... NO TOP HAT???

Like a turtle without his shell, rock-n-roll god Slash hit the pool in Ibiza this week ... WITHOUT HIS TRADEMARK TOP HAT!!!

The former Guns N' Roses guitarist is vacationing off in Spain with his family ... just like they did back in 2012. Dude looks good.

Meantime, here's a photo of Axl Rose at a yard sale in New York this weekend. He looks ... fine?