In my houseplants, I have trouble with mold on the surface of the potting mix. I get this every time, and I do water sparingly. As a matter of fact if I cut back enough to keep the surface dry the plants wilt. My house humidity stays about 50% and no where else in my home do I have problems with mold(Been told I'm a bit of a neat freak by my wife) and I'm just about to toss plants out the door. I also worry because my wife's grandson has mold allergies. He has stayed here for a week at a time and no problems but, this is nasty.
Hydrogen Peroxide works great for this. I use to have the same problem on a few houseplants and also have mold allergies.
There are quite a few posts about this on here but I found the best method that works for me is - 1 gallon of water and about 1/4 cup of the regular household peroxide.
When I get this on the surface of my houseplants I carefully break up the soil with a fork, leaving the soil "fluffed up". If mold forms again, I spray a bleach/water solution on the soil only. I mix this at a rate of 1 pt. bleach to 10 pt. water. I think a lack of air movement is the culprit. Once I put my plants outside for the spring/summer the mold stops forming. I'm sure there's other solutions for this problem out there. This has always worked for me, but I would imagine some plants could be damaged by the bleach solution. Haven't noticed any damage though.
Forgot to mention above, but if the layer of mold is pretty solid I just scrape it off and throw it out. Top dress with a very loose potting soil (with lots of perlite).
Those are some good suggestions if it is mold, but I'm wondering if it might be hard water/salt buildup instead? Typically you won't get mold on the surface unless you're watering a little too much, but especially if you have hard water or if you fertilize fairly frequently and the plants have been in the same pot for a while it could very well be hard water buildup instead. If it's fuzzy though then it's definitely mold, the hard water/salt buildup looks more crusty.
Oh, right ecrane...I didn't think about that. Actually that makes sense since he indicated he gets it everytime he waters. Mold would take a while to grow. Salt buildup disappears when wet and reappears when dry. If that's the case, I guess it's best to scrape the surface and topdress and leach the soil from time to time. Is there a better method of getting rid of the crust?
It might be time to change your potting mix--if it's been in the same pot for a while and you haven't changed out the potting mix it would probably be a good idea to do so. No need to put it in a larger pot unless it needs it, but the potting mix needs to be refreshed on occasion. If there's salt crust on the top, there's also probably too much salt spread throughout the pot just you can't see it, so I'd go for changing out most of the soil not just replacing the top part.
The other thing you can do to help prevent it is to give the plant a good flushing with water from time to time--instead of just "topping off" the plant when it's a little dry at the top, let it dry out a bit more and then give it a really good thorough flushing with water, that'll help prevent salts from building up as much in the first place.
ecrane3 it is fuzzy, so I guess its mold. I've tried stirring the top of the soil and it eventually comes back. The peroxide sounds best. This is miracle grow potting soil and a few others. I wonder if the mold or fungus is in the mix as I have used a sandy potting mix without all the sticks and bark particles. I do water from the bottom up on most of my plants (unless I keep the base to full) so I thought that would fix things.
I'd still try letting things dry out a bit, I've never had trouble with mold unless things were a little too wet. The hydrogen peroxide should work to kill it off, but if you keep things a bit too wet then it can come back.