I'm dealing with some hair algae. As others have said I find there's a bit of a smell when I turn everything off and more noticeable when I pull it out of the water while doing manual removal. It always reminds me of what the beach smells like when the tide goes out and algae, etc. is exposed to the air. A salty, plants, earthy smell. It takes me right back to my childhood when we lived near a beach.

That's probably what you're smelling. I run carbon so I'm not sure if it would be very noticeable without the carbon. When everything is running I just smell the salt if I put my nose right over the tank. I don't use a skimmer so have none of those smells.