It is called paint. If you would watch the games live as opposed to on your DVR, you not only get a better feel for the game, but you can put things into context. For instance, you may have noticed it was raining very hard for much of the game. To mark the lines on the field, they use a white substance. Also, for games of certain magnitude(championships, playoffs etc etc..... something Tannehill fans are not familiar with) they use even more of the substance to put things on areas of the field aside from just marking lines. When it rains a lot, such as it did during the game yesterday, that substance gets everywhere. You may have noticed while watching scoring plays on your DVR, that Gronk was so heavily covered in it, it made it appear as if it were snowing at the game. It clearly was not snowing, given that the temperature was over 50 degrees at kickoff. That could be the substance you mentioned.
If the substance were a red looking color, it could have been the paint from somewhere on the field, or the reddish looking hue left on the hands of players that the football will leave during heavy rain. Terry Bradshaw demonstrated that during the pregame of the NFC Championship and the substance was still fully visible on his hand during the halftime show of that game.
Nevertheless, I suggest you try watching the games live. It gives you a much better perspective as opposed to just watching individual plays and "breaking down the film" when you "put the tape on". If you watch them live, you may actually notice things like flow of game, situational factors that lead to certain plays, momentum, and clutch factor. If you watch a 3rd and long after hitting buttons and just watch for footwork and all of that jazz, you may fail to realize that its even 3rd down and a check down is not an optimal decision given the circumstances. It might be something you mark as a negative play as opposed to "OMG did you see that read progression and location of the open man!!!! If he would have tried to squeeze that one in there to the WR down the field, it was likely an INT. Great job by the QB!!!"
Im shocked. Anyone who understands the game will tell you that Andrew Luck has huge balls. Certain other QBs do not. Tom Brady does, but perhaps that is something that comes with age.
We're on to Seattle.