Anything on the underside of the affected leaves, like rust coloured spots?

The brown spots show through on the underside but nothing more than that.

Nothing is 'wrong' with your Rhodie. It's beautiful and full of buds for next spring. They all tend to shed a few leaves now and then. That's just what they do, but the new growth you have more than makes up for it. Snip off those moldy leaves and get them out to the trash. Keep it well watered and consider adding and inch or two of pine mulch - either needles or shredded bark.

If, as it looks to be, your rhododendron is in a pot it might be time to repot this autumn.

In that case, its leaf spot, not rust, but its of fungal origin. Seems only to be on the older leaves, but you can try a fungicide such as Bayer Fungus Fighter if you like, more information in the link below, you'll need to copy and paste into your browser -

https://www.rhs.org.uk/Advice/profile?PID=573

You don't need fungal spray or anything at all. Rhododendrons naturally cast their older leaves (just like you shedding dead skin cells or hair follicles.) The older leaves die off and decay. Just cut them off so it looks better and the mold doesn't spread. The sprays won't stop this and it shouldn't. If this is in a pot, consider a larger pot or in the ground, when it goes dormant. If you choose a pot, go shallow & wide for the root-ball.

Thank you all for your comments and advice. I'm glad there is nothing serious to worry about.