Why is my pilea stem turn black?
The stem of the Pilea peperomoides does turn a darker shade as the plant matures, but your photograph appears to show a stem that is almost black in colour. If it feels soft to the touch, and looks like it is thinning and collapsing, this is likely a symptom of root rot, which will show up in the plant’s stem, usually darkening from the base upwards.
Root and stem rot are a fungal disease which can take hold when there is a combination of overwatering and poor drainage. Overwatering is one of the main causes of houseplant death. Does your pot have drainage holes in the bottom? Good drainage is a necessity for all houseplants, not just Pilea.
If your plant’s stem feels soft, you could remove the plant from its pot and examine the roots. If root rot is the problem, you will find that the roots too are soft and “mushy”.
Root and stem rot generally cause plants to die. Here is a previous Toronto Master Gardeners post that explains this issue clearly: https://www.torontomastergardeners.ca/askagardener/pilea-peperomioides-leaves-falling-off/ . If this is the issue, the best solution is to dispose of the plant and the soil, and start afresh. Make sure that you dispose of the soil in the garbage, because soil-borne fungal diseases can remain in soil for a long time, and can affect the next planting. Wash the pot thoroughly if you are planning to reuse it – but if there are no drainage holes, that pot is not a good candidate for a houseplant. Here is another Toronto Master Gardeners post about this plant, including some tips for care, and also for propagating new plants from cuttings, which may be of interest: https://www.torontomastergardeners.ca/askagardener/black-leaves-on-my-pilea-peperomioides/