One reason for transient pale skin all over the body in a young infant is
bradycardia (slow heart beat).  Is there any reason to suspect this baby may
have a heart defect, accounting for failure to thrive as well as transient
paleness?  Or may simple bradycardia with no heart defect?

Kathy Dettwyler, writing from personal experience as the mother of a baby
who had bradycardia, not writing as a medical professional.  Peter used to
turn as white as the sheet he was laying on.