Is this mold?

Thanks eggyones

maybe topped the plant poorly?

See grumpy granny, not hard to guess in 1. A diary helps people help you on more challenging issues.

Sadly it will be due to the high moisture level and cold. Even 35-40% RH would me max for that temp. If you can read temp and humidity then go read up on VPD and use a chart. Environmental conditions are my answer of the week. Day and night have ideal targets at different stages of growth. If both are extremes it will stress the plant leading to sadness. Airflow is important and with lights out also, not enough fan to cause wind burn but enough to shake things about will help and a bit of heat? Plus extraction of grow area needs to be calculated and fairly good for night. This makes warming the tent air pointless, so you need warm dry air going in through air intake from outside room. At a higher temp and only guessing on stage of plant 20c you'd want maybe 55-60% max at night with a day of around 28 at around 70%, you'll see when you know there is a narrow corridor on a graph that is ideal VPD throughout grow. Challenge is making the environment. I'm sure someone will say how to treat, but here's how and why. Good luck.

The humidity is at a maximum of 86% at 16 degrees when the light is off

Hard to tell from just a photo... a diary would be helpful.

What is the temperature and RH during the night cycle?