Ilya Samsonov made 22 saves for the Capitals (26-15-9), who have lost five straight at home.
"Last couple weeks here we haven't played with that flow that we did earlier, like the mojo like we used to have here," forward Nicklas Backstrom said. "I feel like we are putting ourselves in bad spots, D-zone, neutral zone, we are not creating as much as we should. I just think that everything needs to be better."
Capitals forward Alex Ovechkin had an assist but has not scored a goal in his past six games, leaving him one short of reaching 30 for the 16th NHL season.
Formenton scored his third shorthanded goal of the season. He fed Brown on the rush, took a return pass in the slot and scored on one-timer.
"Being on the power play is where we want to be and make a difference, and it works against you," Capitals coach Peter Laviolette said. "Right away you're digging out of a hole. So not the start we were looking for with regard to that."