Hee hee, nice toenails, purplestar! ;-)

Ah yes, dried algae. It's not harmful to the plant at all, it just looks ugly. I find it is easier to remove on a rainy day when it has had time to soften up in the rain. Just briefly getting it wet isn't always enough, although the big fibrous chunks should come off pretty easily as you describe. I've always thought it would be interesting to try and make paper out of that stuff!

You're right, you don't want to scrape it off with your fingernail or anything, that will leave marks. A damp rag or sponge might help, or if you can get a hose shut-off nozzle that will close down to a very fine but fairly hard spray, that should help blast some of it off too. I sometimes swab the leaves with a cotton ball soaked in alcohol if I'm trying to remove scale or mealybug, and that seems to clean off any other deposits on the leaves as well. This is safe for the plant as long as it doesn't get really direct sunlight while you're doing it.

If you can keep your cups flushed out with fresh water all the time you can keep the algae from growing (and the mosquitos too). This is a lot easier if you just have a few plants than if you have hundreds.

Hee hee, my toe nails always want to get into the act ;-)
Thanks Lisa, I will give alcohol a go, 2 for me and one for the broms ;-)
It seems that after the easy papery residue was removed, there may have been some hard water residue left where the wet algae was sitting then evaporated, leaving behind a scaly hard water deposit, like the salts that build up around the rim of pot plants. This is very hard to remove.
I will try the alcohol trick. Oh I just thought of something! What about a de-scaling solution like they use in irons and kettles? Maybe vinegar too?
Hee hee, I'm off to experiment!
Thanks Lisa! :-)