Very pretty, Merry. And don't worry about the waterfall scum; it's a part of nature.
And, yes, you should be removing mulm or other gunk accumulating on the pond's bottom. You can usually net it out. A little left in won't hurt, so just get what you can.
I do a partial water change every Sunday (learned about it here on AP). I let the water drain out via the hose coming from the pump, while I do other maintenance tasks. Then when it's 10%-25% down, I start refilling, and don't even worry about chlorinated water from the city's system, since it's such a small amount.