It's just like fold at the gluteal crease..Is there any other symptoms like fever,pain during touching this or decreased in activity of child??
Are the milestones delayed??
If u don't mind just repost this picture with red coloured highlighted one
Next Steps
If u still suspect something wrong just consult nearby paediatrician once
Health Tips
As of now do not massage or press over it if u are doubtful...
Give proper nutrition..Have an eye over milestone achievement