Riley,
While pantheism cannot be reconciled with Catholicism, there are elements of pantheism (such as the need to respect nature) that can. I would also note that there are many statements which, if interpreted one way, are completely and flatly heretical, but which if interpreted a different way are theologically correct.
The first step will be to get your hands on one of Fr. Stella’s books and read it. You need to do that to make sure that you are reading what he has written and not what other people have said he has written. (There is a serious difference.) You need to make sure that what he says really is heretical and cannot be interpreted in an orthodox way. I suggest that you get a few other well-educated people to help you in this effort so that it is not just your personal opinion.
If you then find that Fr. Stella’s statements are indeed definitely heretical and feel that God is calling you to alert the Church about Fr. Stella, the first step in this is to write to his bishop. I am sure that the bishop has already received hundreds of complaints about Fr. Stella, most of them vague and abusive, or at least dealing in generalities and filled with negative emotion. If you want the bishop to take your letter seriously, you will need to be very specific and factual: “Fr. Stella has said this” (and give book, edition, chapter, page, and line number–any investigator will have to check your facts, and you want to make it as easy as possible for him) “which I think is in contradiction to the Church’s teachings in that way” (and again, you will need to cite Catechism paragraphs, Council documents down to the line number, and so on).
Before you write to the bishop, you may want to write to Fr. Stella and express your concerns. Again you will need to be very specific about exactly where you think his book contradicts Catholic teaching and how. You should also invite him to explain to you how he reconciles his thoughts with the teachings of the Catholic Church. If he says “The Church doesn’t actually teach this, but she should,” then it would be good to write him back and ask him to repeat this to you for you to quote him in public. (Private correspondence should stay private.)
I wish you well in your endeavor.