JOIN USBarcelona draw at title rivals Valencia
Barca were denied the opener as Lionel Messi's tame shot after half an hour was spilled by goalkeeper Neto and bounced over the line before the Brazilian dragged it back over but referee Ignacio Iglesias Villanueva allowed play to continue.
The Argentine forward and his team mates remonstrated with the officials to no avail but still had a big say in the game, laying on a perfect pass for Alba's 82nd minute leveller.
Spain striker Rodrigo Moreno put Valencia ahead on the hour, sneaking ahead of Barca defender Thomas Vermaelen to score from close range before putting on an orange wig in tribute to the club's late president Jaume Orti who passed away on Friday.
Barcelona lead the standings on 35 points, Valencia have 31 while Atletico Madrid are third on 27, level on points with fourth-placed champions Real Madrid.
Before kick-off, Valencia paid tribute to Orti, president during the club's last two Liga title-winning seasons in 2002 and 2004, and an electric atmosphere gripped The Mestalla.
Marcelino's revitalised Valencia side were searching for a ninth consecutive league win to cement their first title tilt in 14 seasons but Barca took command in the first half, puncturing the atmosphere with long periods of possession.
Still, the leaders failed to create any clear chances until the game's flashpoint when Messi celebrated his 'goal' only for Valencia to break straight down the other end and almost open the scoring when Simone Zaza flashed a shot just wide.
La Liga is the only one of Europe's top five leagues not to have goal-line technology, although it will implement the video assistant referee (VAR) system from the start of next season.
"La Liga has the best players in the world. It's the best league in the world and it deserves the best technology. The goal was over the line, but neither the referee nor the assistant saw it," said Barca midfielder Sergio Busquets.
Valencia grew in confidence after the break, pressing high up the pitch. Zaza shot wide again and Barca defender Nelson Semedo gifted the ball to Rodrigo, who failed to capitalise.
The striker found the target soon after, poking home to complete a sweeping move between left back Jose Gaya and winger Goncalo Guedes, putting Valencia on course for a first home win over Barca in over a decade until Alba struck.
Elsewhere, Sevilla battled back from two goals down to win 3-2 at Villarreal in the first game since the news that coach Eduardo Berizzo has prostate cancer. Fifth-placed Sevilla have 25 points and are now four ahead of Villarreal in sixth.
Seventh-placed Real Sociedad drew 2-2 at home to second-bottom Las Palmas, while Athletic Bilbao were held by the same scoreline at fellow strugglers Deportivo La Coruna.