You are seeing cyanobacter growth in the sand bed. It is normal, expected, and indicates the bed is functioning normally.
Let it be or marvel at the neat colors...
This is another way in which the bed functions in metabolizing excess nutrient. Later other critters will eat the cyanobacters, and mobilize the nutrient so that it may be exported. When the cyanobacters are eaten, the nutrient re-enters the sand bed to feed the biological filter and eventually to leave the system as gas, or as some biological export such as excess algae.
There are NEVER any dead spots in a sand bed.