What should I do if I’m praying while being slightly deviated from the qibla? Should I continue my prayer in congregation at the surau or is it invalid after I realized that the qibla is wrong?
Facing the Kaaba (the qibla) means a person facing the qibla with his chest, not his head for those who perform the prayer while standing and lying down; with his head and chest for those who pray on his side; and with his head and toes for those who pray in a supine position. 
According to Syeikh Prof Dr Muhammad al-Zuhaili, for those who are far away from the Kaaba or hindered from looking straight at the Kaaba, his prayer isn’t affected if the position is slightly to the right or left from the Kaaba, for it is a mistake that happens because he is too far away, as long he hasn’t deviated away from the Kaaba. 
If he is slightly deviated to the right or left, then it doesn’t affect the validity of the prayer. It is best to utilize the service of the Falak Unit from the Mufti Department of each state or NGOs who offer the service of calculating the qibla to get the most accurate qibla direction. Wallahu a’lam.