I have a mountain bike and when I put some power on the cranks the chain slips.
It’s only momentary but usually with every stroke under power.
I have checked the chain line by eye and it looks straight to me (the untrained eye).
Hard to see with the pic from behind, but I’d be ditching the tensioner and finding a gear that works without one… Tensioners suck… I built up a SSMTB a while back without one and loved it… Just didn’t ride it enough so it went.
take a link out of your chain and it won’t slip anymore. or put a bigger rear cog on. nice cranks!
Adjust the chain tensioner loop… Chain isn’t slipping just contacting the loop
This is your old trek? You sell it.
Shorten the chain, get a dmr sts and run it in the ‘up’ position.
How do I adjust it please?
With a 8mm spanner on the hex nut
This. I ran one on my old mountain bike and it ruled.
‘Magic gears’ can be a PITA. 
I’ll try this tomorrow…thank you boys!
The chain is slipping because the tensioner is pushing the chain down and limiting the engagement between the cog and the chain. This is a common problem with this type of chain tensioner and generally speaking even if you remove a chain link the problem may still exist.
The best option in my opinion/experience is to use an old rear derailleur and stretch the cage along the chain stay of the frame to ensure maximum tension on the chain and an upward push on the chain to give maximum engagement.