Not heresy but perhaps illicit, and could be invalid if the words of the Consecration were changed sufficiently.
My priest ad libs the Eucharistic prayers. He doesn’t use (and I have checked every single solitary available ‘approved’ Eucharistic prayer there is) any approved prayer. What he does it take a few snatches of one (say Eucharistic prayer 3) and say 'from the rising of the sun to its setting" but then go on to talk about the beauty of Mother Earth and Earth’s children (which is not found in ANY approved prayer whatsoever) and then onto things like, "and He blessed YOU, Father, for your kindness, and at the meal he gave bread to His family and His friends and said, “Take this and eat, this is my Body. Receive this as a sign of God’s Covenant” and then he will go on to maybe reference Eucharistic Prayer 2 briefly, with “You give us the bread of life and the CUP (changing from ‘chalice’) of salvation”, and then add in things like, “and so as we gather around your table we thank you for allowing us to be present in the light of your face” etc.
It’s obvious that he knows what the right words are but chooses to use his own paraphrases and tweaks and add-ons because he thinks them ‘superior’ (he once told me not to pay any attention to ‘the song book’ because ‘here we are open to spontaneity and freshness’–although how it can be fresh when for the majority of the time he uses the same ‘paraphrases’ over and over, I don’t fathom–) and because if one follows the weekly collects, when he does a particular collect for a Sunday he uses one or two words or even a sentence FROM that particular collect but then adds on a completely different phrase or sentence that gives an entirely ‘different’ perspective. For example, if the collect had anything about a need to be humble that wouldn’t be mentioned but there would be a phrase about how thankful we are to God for His making us “Holy and chosen and beloved as we are”, etc.
There are so many real, true ‘options’ in the Mass and opportunities to choose (Penitential rite 1, 2 or 3? Apostles Creed or Nicene or baptismal promise renewal? Which setting shall we choose for the Responsorial Psalm? Which Preface? Which Eucharistic Prayer? Sing or Say the Our Father (or Gloria, or Sanctus, etc.), not to mention the entire homily, so I truly, truly do not understand why a priest feels the need to ‘rewrite’ the things in the Mass which are specifically written to be said in exactly ONE way. It’s just --mindboggling, especially in this day and age.