Hello Helene,
I had the same spots on the head of Apistogramma. At first fishes with spots showed no changes in their behaviour but some weeks later they became apathetic and died! I searched in the WWW and found the “black spots disease” caused from Trematods or cancer or Tuberkulose, others say that the spots are caused from dissolved organic material. This phanomenon is widespreat among cichlids, perhaps you find something useful in cichlid forums.
I hope the spots are not the same as on my Apistos.
best wishes and healthy fishes!