What's your question? Ask

Could you include a photo here? This will help me. It could be a flower, but I won't be able to tell without a photo. Here is an article that will help in the meantime: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm

This is a fungal infection. Usually this is caused by overwatering. You should only water when the top two inches of the planting material is almost completely dry. You can attempt to correct the issue, as long as it isn't too far developed. Mix peroxide in water. Just replace 1/4 of the water with peroxide. This will kill off anything that doesn't belong there.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm

Uh-oh... This is a fungal infection that will lead to the quick death of the plant if not treated.

The first thing that you can try is to mix 1/4 peroxide with 3/4 water the next time that you water. This should only happen once a week for 2 or 3 weeks. This will help to kill of the infection. The infected material will not go away, but the spread will stop.

Make sure that this plant has PLENTY of time to dry out between watering, or is in the proper soil mix that will allow it to dry out in the correct timeframe. This problem is usually caused by overwatering.

These should become completely dry throughout the entire container before watering again. I recommend a succulent or cactus soil blend to help with this. A container that let's out all extra water is helpful as well.

This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm

Fungal attacks are extremely difficult to stop. The best option is to find any uninfected stems and re-start a new plant from them and throw the rest of the plant away. Fungicides are available that could slow down the fungus attack, but the fungus rarely is eliminated in this way.

We don't recommend repotting for a couple of years after starting a cutting.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus-cuttings.htm

It appears to be one. I would recommend using wettable sulfur and DOLOMITIC lime together to correct the issue.

If that doesn't stop the spread (old spots will not disappear, but new growth will be clean when it starts to work) then you may have to turn to a more harsh copper fungicide.

This article will help with the care of these plants: https://www.gardeningknowhow.com/ornamental/cacti-succulents/night-blooming-cereus/night-blooming-cereus.htm