Okay so i got two Hoplosternum thoracatum in my tank and they're doing fine until just recently i discovered that they started to have a stripe and markings on the head which were significantly lighter than the rest of the body.
Can anyone tell me what is happening, they're acting normal but can't take a photo since they're hiding under the rocks or driftwood as always.

One hour and 50 photos later, this is the best one i could take after feeding time;