Mold spores are everywhere, both inside and outside our homes. This is unavoidable.The reason we see active, growing mold in any place in our homes is that sufficient moisture, temperature and food are present for growth.
My theory is the water evaporating and transpiring from the pots and plants may have tipped the balance slightly in favor of mold growth - a local 'micro-climate' above your sink.
If you are able to move the plants into an area further from the window with good air circulation and
- clean the walls with an anti-fungal, like bleach,
- then you can observe if the mold returns.
If it does, then there may be some building related items to consider, such as condensation occurring from a broken seal on an insulated window or an external moisture issue relating to window flashing or sealant failures.
Best of Luck,