Coffee grounds are, indeed, a good thing.

As with anything organic - moisture and warmth will create mold. I would suggest mixing it with your potting soil throughout, rather than layering it on top. It can also form a crust when dry, which might not allow your soil to breathe properly. Used indoors, I think it's best as an amendment to the overall soil.

The majority of folks use grounds outside in the garden as a soil amendment (not a mulch) - or they compost them first.

Just wondering if I should mix it into the potting soil instead????

Whether coffee grounds are a good thing for any soil depends on whether that soil has the Soil Food Web to make use of the nutrients in the coffee grounds. Most potting soils do not because most potting soils are peat moss, coir, fine pine bark, with some perlite/vermiculite added to promote drainage. There is little, if any, bacterial activity in commercial potting soils so there is nothing in those potting soils to utilize the nutrients various forms of organic matter have until the bacteria these forms of OM do have to digest them get to work doing that. Whether mixing these into the soil will do much more good or not depends to on what that potting soil is made of.
That mould growing on your coffee grounds is an indication of a fungi trying to digest those grounds and make the nutrients the grounds have available, but the presence of these molds may also indicate that you are keeping that potting soil too moist.

Dog, for most houseplants, you do not need to add anything to the potting soil.

I'm sure you added the coffee grounds in an attempt to either give some acidness to the soil or possibly to thwart some bugs that might like to invade the soil....such as soil gnats.

If you understand just why your houseplant is advised to use potting soil...and not garden soil, then you will understand that potting soil has everything in it to feed your plant for a time before the need to give outside food.

Potting soil is "sterilized" to keep out such bacteria that could harm tender plants....including seedlings.
By adding material that may defeat this "clean" mode of the soil, then is going against what you used the potting soil for.

And, depending on just how much coffee grounds you gave the surface, such small amount of soil in the pot can easily have its pH taken to maybe too high a point that is not doing the plant any good.

Any good houseplant potting soil has already in it what can feed the plant sufficiently in its young age and promotes proper growth.

The trick though, is buying what is "good potting soil".
There's a lot of garbage out there calling itself potting soil. but adding coffee grounds does not do whatever soil any good.

After the plant has used what nutrition the potting soil initially had, then there are countless proper fertilizers on the shelves for our plants.

You didn't do anything wrong. The mold is only a bad thing because it is probably distasteful to you to have it inside the house. If you wish to use coffee grounds for your house plants it will probably be a good idea to gently scratch the grounds into the surface of the potting soil and/or cover with some appropriate mulch for your indoor houseplants.

As to potting soil and how much of a food web it can support.
Potting soil has to support some microbe life, at least after it has been out of the bag for a while. How do I know this? Because Miracle grow works for container plants in potting soil. Miracle grow doesn't work for plants in a completely sterile media like in hydroponics, most of the nitrogen in MG is urea or amonia nitrogen which requires soil microbes to change it to a plant usable nitrate form. This is why MG alone does not work as a hydroponic fertilizer.

Anyway, if the houseplants have been in their potting soil for a while, they could probably use a little weak feeding and it is quite likely that there are some microbes in there to help. However, I don't know how to get the other nutrients needed to your plants since coffee grounds are primarily a nitrogen source.

I always thought mold grew on the coffee grounds when they were too acidic to be applied

Molds will grow on coffee grounds, or any other oganic matter, that is kept too wet.

Ok i dint know anything about what the answer's some of you guys gave but I know a little about growing things so Dog, I use coffee grounds on my indoor house plants and I use potting soil aswell let me just say its one of my best tips/tricks it does wonders for my spider plant, devils ivy, bouston fern, and a fee others. So I'm not sure why I just know my grandmother knows what's she's talking about. She's very wise when it comes to these things

Jessyka- are you doing some search for these topics? this one is 13 years ago.