There's really not enough information to make an educated guess. Not enough detail to really pick anything out.
Perlite does not decay and is chemically stable - meaning it won't turn colors on its own without extreme heat or other radical process.
Perlite is porous and can become inhabited by micro-organisms ( fungi fall into this category ).
Your perlite is obviously stained by something. Is that "stain" alive? or residue?
Was the perlite clean when you started? Was it new or cleaned from a previous grow? Did you rinse it out prior to using it in your terrerium?
If it was old, how did you clean it? Where did you store it?
Until you know, treat the project as being contaminated. Wear gloves and respiratory protection. Pink is a "red flag" among contaminants - they can be damaging to humans.