The white cottony stuff is fungal hyphae. In this pot I found a few of what appeared to be fungus gnat larvae, some of what I think were predatory mites, and other creatures such as springtails. The greenhouses were these plants came from also had thrips and could have had broad mites. I thought I could figure out what it was from what keeps it from spreading. That sulfur helps isn't much of a clue.
Sulfur is useful to combat mites and fungus.
These are normal leaves The red leaves are damaged. I think the damage shown here is a fungus that spreads during the winter when the temperature is cool and only shows an effect when it warms up in the spring. It could possibly be a Botrytis species. I now spray with Cleary's 3336 as soon as I start seeing the cottony growth near the apex of the plant or the leaves starting to turn red. Cleary's 3336 is generally safe to use but don't spray plants that don't have problems.