What bug is this one?

[rbarata]

It has around 1 mm in length (0,039 in) and it reacts to movement of light by hidding fast. It moves relatively fast, considering its size.

[PaphMadMan]

Always a challenge photographing something so small clearly enough to identify, but they appear to be some type of springtail (Collembola). There are many types but in general appearance this is pretty typical.

They usually do no direct harm but are often a sign that media is breaking down, and they can spread bacterial or fungal pathogens.

[rbarata]

It's a challenge, indeed.
Also, the bug hides as soon as I try to photograph it.

The medium has today approx one year but as it's a Zigopetalum I'm keeping it on the moist side. Probably high humidity is the right condition for springtails to grow.