Head slap?
 

I have three videos showing the left tackle for Alabama seems to slap the helmet of the defensive end. Is this legal?

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The first two are definitely not legal. The third seems to be lower and may not be a foul.

I think he's low on the first one too, and can't tell in slo-mo on the 2nd, as the view of the arm is blocked. The third is definitely on the back, not the helmet.

Wish we had the opposite angle.

To answer the OP, if any of these contacts are on the helmet, it's definitely not legal. If they are on the shoulderpads, it's legal - it appears he's just using the defender's momentum against them.

I will say the 2nd one is more likely guilty than not guilty - just can't be 100% sure.

Agreed. This is a common technique used by interior linemen on screen plays. I had an observer tell me one time to flag it as an IBB when they hit them in the back, but I disagree. If there is contact with the helmet I may consider a warning the first time since it is easier (and probably more effective) for them to push the body.

It's hard to tell in the first two videos if he made contact with the helmet. The third one looks like shoulder in slow motion.

Would you consider #1 a "striking blow"?

It's no more than a swim move that happens to coincide with the defender's momentum. Many blocks are "striking" to the same extent that this one is.