I do, and here's how I do it. I take the old potting soil, put it in a 3 gallon pot, and pour a kettle of boiling water through to kill it, then I mix it half and half with new potting soil. Things grow very well in this, and I've never had any root rot problems from the old soil.
Didymo (Didymosphenia geminata), also known as rock snot, is a non-native invasive microscopic algae (diatom) that can produce large amounts of stalk material to form thick brown mats on stream bottoms.
Didymium is not a name you will often find in textbooks these days. It is the name of a pair of elements which lie next to each other in the lanthanide or rare earth series - what used to be the Wild West of the periodic table. The fourteen elements that constitute the series are remarkable for their similarity.