Possibly one of the common myxomycetic slime molds. If so, a relative that most often seen in our gardens & beds that raises plenty of eyebrows is referred to as the 'dog vomit fungus', though we now know it not to be a true fungus.

Al

What do I do about it? Also, looked for information about slime molds online...seems like they are usually wet? This is very dry. The house is extremely dry, too, with the heat on and since I'm in a bone-dry desert area anyway.

They may come together and mimic a single organism when the cultural conditions are favorable (moisture levels, temperature, etc), but when cultural conditions change, they may leave you with the remains you're seeing on the surface of your soil. I wouldn't be too worried about it unless it bothers you, in which case - just scrape it off as it occurs.

Al

Well I may not have the expertise of some others on this, certainly not of our friend Al, but I can suggest 2 things (w/ which I'm guessing he'll agree).

If it were mine, I'd change the mix & ALWAYS use a pot w/ a drainage hole. I understand if you wish to keep the look of this white ceramic pot w/out a drainage hole. In that case you could plant in a smaller plastic pot w/ holes & insert that into the larger white pot. This would make the white pot a cache pot, that is to say the outer, decorative pot.

I too am in heated place (NYC w/ radiators, steam heat). I always use pebble trays under my pots & never have a mold problem.

When I read the OP, I read it to say "I have a begonia in a ceramic pot WITH a drain hole in the bottom". Karen's advice is correct and is something I too would have suggested if I had read your post correctly/more carefully.

It's extremely difficult to keep a planting w/o a drain hole in the primary pot from continual decline, and you can't do it using untreated tap water and/or soluble fertilizers. Like Karen, I think you would be well-served should amend your ways and adopt a durable, well-aerated medium. Using holey pots will also help to eliminate prayer as a significant and requisite part of your growing regimen. ;o)

Al

I have had a growth exactly like this for a year or so now on the soil of my easter cactus. It is like a forest of tiny ivory eggs, maybe a couple tenths of a millimeter wide each. The growths are very hydrophobic. I was wondering what it was myself. I generally just ignore it, but it's a bit irritating for watering.

Tapla, I couldn't find a similar slime mold on Google Images. Do you know/guess what the name is by any chance?

Color? Has it changed color? Does it move around as though it was an animal? Look at pics of Fuligo septica and Physarum polycephalum to see if it's either. If not, try adding your own description here. There are a few others better qualified than I in the area of IDing this sort of thing.

Merry Christmas!!!

Al

Well...update a few months later... I will amend my ways and not plant in un-holey pots (it was just that one plant, but good advice nonetheless). The stuff hasn't changed at all, so I decided to ignore it and let it do its thing. Thanks for the advice and info :)