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Starfish cactus, botanical name Stapelia, is not a true cactus but is actually a succulent in the family Apocynaceae. Most advice on caring for cacti applies to it too, however.

Repot the plant by carefully removing it and the media surrounding its roots from the pot, following the advice in this article:

https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-repotting-info.htm

Afterward, be careful not to overwater it, since this can kill succulents quickly.

Use a potting mix designed for succulents and cacti, or one you make, as described in this article:

https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-potting-soil.htm

It is unknown whether the starfish cactus (Stapelia grandiflora) is toxic or not, but it does have relatives that are toxic. It might be best to keep the plant out of your grandchildren's reach.

Without seeing it, this sounds like powdery mildew. Neem oil will get rid of this, as will rosemary oil, or garlic extract. Mix one or more of these in water and spray the plant. You may spray the growing medium as well. Here is an article for more information on powdery mildew: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

As a general rule, add water when the top half-inch of soil feels dry. When watering, moisten the soil thoroughly, allowing it to soak up the water, and if it does so quickly, add more water until it comes out the drainage holes. Most cacti need to be watered once a week.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/star-cactus/

Beautiful plant! I would try to identify the type of fly. Are they fungus gnats?
This article may help.
https://www.gardeningknowhow.com/houseplants/spider-plant/spider-plant-fungus-gnats.htm

You can grow the seeds, but stem cuttings are faster and you get a plant that is true to form.

Start the seeds in a seed tray filled with cacti potting mix. Lightly cover with soil with potting mix. Keep seeds warm and mist often to keep soil moist. When the seedlings are about 2 inches tall, move them into individual containers and water when soil becomes dry to the touch. Continue to care for them about a year before moving outside. Harden them off by increasing the light from a window, more each day. Plant them outside in part shade or with some morning or evening light. Do not give them full afternoon sun.

https://www.gardeningknowhow.com/ornamental/cacti-succulents/starfish-flower/growing-starfish-flowers.htm