Yes, melamine foam (aka magic erasers) removes marks via micro-abrasion, so you are basically just sanding them.
If you live in a mold-prone environment and wish to seal your ponies, I'm afraid you'd have to do more than just wiping them with almond oil. (Your ponies' pores would be impregnated with rancid oil a year down the line. I wouldn't recommend it.) Using a dimethicone-based product, like some hair conditioners, would be a better bet, but would leave your ponies fairly glossy and the other substances in hair conditioner might react badly with the vinyl over time.
If you want to get serious about it, first you'd have to sterilize the ponies, otherwise you're just sealing existing spores into the plastic, then you'd have to apply a product that's made to protect vinyl. You can often find these for use on cars/boats. I don't know what's available in where you are, but the brands I'm familiar with are 303 and Poorboy's World. (My uncle restores vintage cars which is how I know about them at all. I do not use these products on my own ponies.) Keep in mind, you might have to repeat the application process every so often, which may eventually cause damage to your ponies in itself, although not of the mold variety. I don't know what the long term effects on ponies' paint would be, for instance. It's up to you whether it would be worth it or not.
Sorry for the essay, but I hope that helps you a little.
![Blush :blush:](https://mlparena.com/Smileys/default/redface_001.gif)