>> the white balls were a little firm and didn't dissolve in water.
>> this morning I moved the pot and it seemed like there were just as many, if not more white balls with new little orange ones popping up.
That makes me think "fungus" even more. There must be something for them to eat, either organic components in the soil, or some organic fertilizer.
(Touching a match to one might distinguish between Styrofoam and fungus: both would stink, but I would expect Styrofoam to flare up with oily black smoke that smells like petrochemicals. Or drop one into a little mineral spirits. Or drip a drop of gasoline onto one. Those would dissolve Styrofoam, I'm pretty sure. I wonder if slicing one open would reveal a mushroom structure?)
Yeah, I would scrape them away and replace the surface mix. Then think about encouraging a drier soil surface. If you ever re-pot the tree, maybe consider a less-organic potting mix, maybe one that holds a little less water.
If you have a lot of pots indoors, and the fungus only appeared when you set one small pot on top of the potted tree ... don't do that. It hints that something is different about the small pot - either it has a lot more fungus than anything else, or the runoff from it contains things that encourage fungus.
I would put that small pot outside since it "infected" a valuable plant, or at least scrape the surface and flush what's left a few times (i.e. flush out soluble organics).
You can flush a pot quicker and more thoroughly (with slightly less risk of drowning roots) if you set it on top of some toweling, old cotton socks or other absorbent fabric and let on flap dangle down 6-12 inches. The dangling flap is a wick that draws dirty water out of the pot, encouraging the mix in the pot to let some air back in between flushes.
Or take the small pot outside and re-pot it with clean soil-less mix. Maybe most soil fungi are not harmful, but as an undesired hitchhiker, it's a "weed".
