JuJu Smith-Schuster and James Conner were both fined by the NFL for violating its sock policy in last week's game against the Titans.
Smith-Schuster posted a picture of the letter he received from the NFL on his Instagram story on Tuesday. He was fined $5,000 for his socks not covering his lower leg:

He followed up with a picture of the letter Conner received for the same violation, saying "Don't worry y'all, James got fined too" with a laughing emoji.

The issue is covered in Rule 5, Section 4, Article 3 of the NFL rulebook in the item labeled "stockings."
"Stockings must cover the entire area from the shoe to the bottom of the pants, and must meet the pants below the knee," the rule reads. "Players are permitted to wear as many layers of stockings and tape on the lower leg as they prefer, provided the exterior is either a: (a) one-piece stocking that includes solid white from the top of the shoe to the mid-point of the lower leg, and approved team color or colors (non-white) from that point to the top of the stocking; or (b) solid color stocking (i.e., entire stocking from bottom of pant leg to top of shoe), but all players must be in the same stocking style and color in any particular game. Solid stockings must be a consistent color from the bottom of the pant leg to the top of the shoe. Uniform stockings may not be altered (e.g., over-stretched, cut at the toes, or sewn short) in order to bring the line between solid white and team colors lower or higher than the mid-point of the lower leg. No other stockings and/or opaque tape may be worn over the one-piece, two-color uniform stocking. Barefoot punters and placekickers may omit the stocking of the kicking foot in preparation for and during kicking plays."
Smith-Schuster and Conner can provide written appeals for the fines if they wish, otherwise, the fines will go toward the NFL Foundation.