spray with detergent?

Yikes. Since the black stuff comes off, I'd use a wet/soapy sponge, cloth, or papertowel and carefully clean each leave, then rinse the entire plant. Then watch it carefully....

I wouldn't use any surfactant (soap/ detergent/ cleaning product) not specifically formulated to be safe for plants; even then, there is no guarantee what you use as a cleaner would be safe for ALL plants. That many growers protest by way of "I've been using Dawn or Ivory or Joy or Palmolive dish washing detergent on my plants since before the Mayflower landed", the practice still results in harm to plants despite those protests. Ask me about my old friend, Joe Pye, to learn how I cured him of clinging to his "It works for me" arguments.

Even at low levels household cleaning products dissolve and strip cuticular wax from the leaf cuticle (skin), wax the purpose of which is to prevent excessive water loss and a pathway by which disease pathogens can enter the plant. Should the soapy solution get into the grow medium, it destroys root lipid membranes. So the question isn't whether or not it will cause harm, rather, it's: How much harm will it cause?

It looks like Sphaerobulus stellatus, aka artillery fungus. Compare at link. If so, I would bag the plant and dispose of it in the trash, or burn it. If you thought about treating it, you'd need a fungicide, but as far as I know, there are no fungicides that claim to be remedial for artillery fungus.

Al

If it is Sphaerobulus stellatus, from anything I have read, may be unsightly but is not pathogenic to plants. The spores a amazingly long lived and can remain viable for over a decade.

..... and it can be a serious nuisance on surfaces other than foliage ..... like vinyl/ aluminum siding and trim, stone, interior/ exterior walls, .....

Al

I haven't seen this on my crotons but thanks for the post..good to know..

Al..thanks for your explanation about using household products on plants..we're in the minority of people that don't use homemade concoctions on their plants and soil..and they wonder "what's wrong with my plant?" Lol..

Al..heard this one?..spray Listerine all over your yard to keep mosquitoes away?..Lol..

I don’t know if you have a Reddit account, but I made mine specifically for the houseplant and plant clinic pages. The communities are amazing and they’ve helped diagnose/save multiple of my plants!