Make sure it's not soil mealybugs. Just touch one of the spots and see if it moves.

Not root mealybugs, but a mold or fungus of some kind. It actually looks like a species of slime mold, which is neither a mold nor a fungus.

So do I do anything? Or is it harmless. It's a fungus I believe. There was some more of it on some dead plant matter lying on soil.

Wet it down with 3% hydrogen peroxide. Repeat when it returns. After a few treatments it shouldnt come back.

Not sure if it is harmful. But I use hydrogen peroxide to kill any mold that grows in/around my potted plants.

I meant to say, not sure if the fungus is harmful. The hydrogen peroxide treatment does not seem to be.

Of the many thousands of fungi that plants find themselves in contact with, only a few are harmful; so while we can't say for sure there is nothing to worry about, odds significantly favor that being the case.

MG soil does retain a LOT of water, so you'll need to keep your watering hand under control because over-watering is probably the potential limitation you're most likely to encounter as a matter of course. DO use a 'tell', like a wood dowel sharpened in a pencil sharpener & stuck deep in the soil or a bamboo skewer purposed the same, to see if you really do need to water. If the stick comes out wet, dark. or cooler on the inside of your wrist than before you stuck it in the soil, you needn't water.

Are you comfortable with your ability to keep plants happy, or in need of some guidance in that regard?

Al

I do ok with outdoor plants. Indoors, not so well haha. So yeah any advice is greatly appreciated.

If you read this basic overview that will help you avoid almost all the common problems those fairly new to indoor gardening encounter almost as a matter of course, and then move on to gaining an understanding of how water behaves in soils, a light should have gone on that signifies the realization that your soil choice, watering habits, and nutritional supplementation program pretty much determines how close you'll be able to come to providing your plants with the growing opportunities they need to realize their genetic potential. Almost ALL plants have the genetic potential to become beautiful specimens, if we just learn how to eliminate the conditions that limit them; and it's not as difficult as it seems on its face.

Al