The Grinning Man is reported to be human-like in appearance, though is commonly associated with UFO activity and is sometimes believed to be an alien. It is also believed possible that he is connected with the Men In Black. In his first sighting, he was described as being over six feet tall and wearing a reflective, green suit with a black belt. He had a dark complexion and small, beady eyes set far apart. That could fly with his wings. He was described as not having any nose, ears, or hair.  In his second known encounter, his suit was said to be blue instead of green, but still retained its reflective property.  Along with that he was described as looking perfectly natural with slicked back hair, a coat with the top two buttons unbuttoned, and having pants lighter than the coat but still the same material. He was also described as being quite tan, though not dark, and looking like any normal human being.
According to reports made by Woodrow Derenberger, Indrid Cold came from a planet named Lanulos in the Genemedes galaxy, and that there were two other "grinning men" by the names of Demo Hassan and Karl Ardo. 
Indrid Cold was first seen on October 16, 1966, when two boys, Martin "Mouse" Munov and James "Jimmy" Yanchitis, in New Jersey were walking on Fourth Street when they saw a surreal figure standing near a fence. As they walked closer, the figure was a tall bald man wearing a metal green suit who was staring right at them with a huge grin. The idiosyncratic man chased them until they got away from him. UFO sightings were also reported around the area. 
"Jimmy nudged me...and said "Who's that guy standing behind you?" I looked around and there he was...behind that fence. Just standing there. He pivoted around and looked right at us. Then he grinned a big old grin." 
Second SightingOn November 2, 1966, in Parkersburg, West Virginia (around the same area and time as the Mothman came about), Woodrow Derenberger was driving his way home on Interstate 77 until he heard a crash. Then, an unidentifiable vehicle appeared to land in front of his truck. He described it as "an old-fashioned kerosene lamp chimney, flaring at both ends, narrowing down to a small neck and then enlarging in a great bulge in the center." The Grinning Man came out of the vehicle with a dark tan, and walked up to Derenberger and telepathically said his name was "Indrid Cold" and he meant no harm. Cold said he just wanted to know more about the human race and he would visit Derenberger again. After the encounter, Derenberger stated that Cold revealed he was from the planet Lanulos in the galaxy of Genemedes.
Third SightingDuring the same period in Point Pleasant, West Virginia, the Lilly family had been reporting that poltergeist activity in their home, such as diamond shaped lights. The Lilly's daughter, Linda, was sleeping one night, and awoke to see a man standing over her.
"It was a man, a big man. Very broad. I couldn't see his face very well, but I could see that he was grinning at me. He walked around the bed and stood right over me. I screamed again and hid under the covers, when I looked again he was gone." 
ExplanationsIt is commonly believed that Indrid may be an alien entity and is supported by its close connection with UFO activity and with Derenberger's sighting. It is also said by Derenberger that the Grinning Man is in fact a species of aliens with multiple "grinning men". Although, this is unlikely, unless the aliens are shapeshifters, because it would be impossible for a race to take the exact same evolutionary path as us.
However, the third sighting by the Lilly family suggests instead that it may be a ghost or spirit of some kind and was connected with poltergeist activity happening in the residence. It may be a prank that went too far, or a man with a mental illness. Others believe that it is a normal human man or a hoax based off of the popularity and fear of the Mothman, who had been first sighted around the same time.