I think part of the issue is the description - the hint that this was the end of riff 2 helped me iron out what I think you are asking about.
The 5:5 and 5:7 means string:fret as asked. It does seem like the written description is a bit confusing with the direction and string:fret being combined.
For playing, I would say that Justin is simply muting strings 5 to 1 at the last fret (7) he was on. Nothing more special than that.
the transition into the third riff starts with the slide from 5: 5-7, then mute for for 2 chugs on 6:0, then 5:7 and 6:6, then three chugs on 6:0, then the F power chord and pull off. this eventually changes into 5:7, 6:6, 6:5 and two chugs then the F power chord. I see it is written 6:1 and 6:0 to simplify and that is not what I hear - I play the F power chord (6:1 and 5:3 - 13XXXX in the text) and pull it off to go back into the chug on 6:0.
This lesson really is a hassle. It’s very unclear and the fact that the tab is not included even via the app makes it worse. I understand it is a different service, and the website is free, but I’m using the app and I don’t even have the (unclear) instructions on this web page.
If you give an exercise on the app, it should be manageable to do it without paying more via the website (which you need to be aware of)
As @stitch and @TheMadman_tobyjenner said the tab for the riff is available for free in the text below the JG website Module 12 lesson. With the paid tab service you would get the tab for the rest of the song. If you are using the app for lessons, it is always good to check the website lessons for additional information such as tabs for riffs and sometimes pdfs and GuitarPro files for song studies. The text below website lessons is also very useful for lessons with multiple chords, since it includes the chord diagrams.
Well I have to say, I’m having a lot of fun with Enter Sandman.
I’m pleased to see that I’m not the only one having problems with the fingers for riff 1. I am finding it very challenging. I can’t play it without muting some of the strings. I found a work around by putting my 1st and 3rd fingers on those 2 frets (7 and 5) on strings 4 and 5, then taking the first finger off the 4th string and using fingers 1 and 2 to play the fretted notes on the 6th string. It works well for me, but I’m still practicing the way Justin shows it as well, as I understand that it’s an exercise to build up finger dexterity and independence. However, it’s not easy and it hurts my wrist if I’m doing it for long and I think I risk injuring myself. I keep looking at how Justin does it and his arm, wrist and hand looks completely relaxed, but I just can’t do that. I will keep trying.
The flick off from the F power chord is not easy. I’m having problems with that.
The other thing to note about this is that I didn’t find the video lesson for this riff to be completely understandable. Justin goes over riff 3 extremely fast without really clearly explaining it. However, if you go to the video lesson for the full song (it’s a grade 4 song) the riffs are explained a lot better, slower and really step by step. I am using the full song video for this rather than the lesson in module 12. I found it a much better lesson. In that sense, the lesson in Module 12 is a bit superfluous, except that Justin gives some additional advice about trying to play songs that are above your current level, which is nice.