No mites. The central bud fell asleep. It will probably wake up on its own.

Potting the plantlets so deep in a solo cup - or pot - is very detrimental. Limits the light and airflow to the roots. Accurately pick your young plant up, add a spoon of soil - and put it back. Soil should be on a level with a rim. It will settle bit later. I do not know exactly why they react this way on a deep potting - but it is my experience. Streptocarpus plants have young leaf rot if planted this deep.

Good Luck

I.