Yes the discoloration is the front upper right hand corner and perhaps the upper left hand corner, along with the area near the spine on the back cover.
As to slabbing it, that is entirely up to the individual. I personally like to have a graded value of around $100 or more to consider slabbing the book. Other collectors have different price points on this, so ultimately it's up to you. The Go collect site has this book valued at $110 in 8.0, and $70 for a 7.0 copy.
Being a novice, I would recommend you do a lot of studying, specifically graded books of Astonishing Tales 25 for starters, and other books just to gain experience in being able to grade a book accurately. There are numerous examples on Ebay of this particular issue. You could also check out Comic Connect, Comic Link, and heritage, as you will find larger scans you can review on those sites. You might also consider buying a CGC graded copy instead of having your copy slabbed. There are also services out there that will press and clean a book to help it achieve a higher grade.