My indoor Monstera plant has this weird, lumpy yellow growth around the roots, can anyone tell me what it is and how to treat?
These are fungal toadstool growths, specifically, Leucocoprinus bimbaumii; they're not uncommon in houseplant soil, though are usually more likely to appear during summer, and in particular, if the soil mix the plant is in is soil-less. They will cause no harm at all to the plant or you, so long as you don't eat them, so they don't require any treatment at all. You can remove the caps once they form to stop spore spread - if you are keeping the soil in the pot too wet, let it dry out a little between waterings. Hopefully, there is a drainage hole in the pot you've used to allow excess water to flow out - empty out any outer tray 30 minutes after watering.
Eradicating them completely is difficult because these are only the fruiting bodies of mycelium already present in the soil mix - they just happen to be fruiting now, but if you're desperate to get rid of them, you could try changing all the soil, though that's not always successful. More info here Getting Rid Of Mushrooms Growing In Houseplant Soil.
It's probably nothing much to worry about, characteristic of the compost the plant is growing in and perhaps a bit too much watering. Please see if the answers and comments in this similar post are helpful and if so vote up the exchange there as you see fit.