There were no die rolls involved -- except for the saving throw.
This is a crucial issue to the kind of game I like to run. Is this fair? Was the death due to player choice or factors that were outside of player control?
Player Agency: There are several factors regarding the discovery of the mold.
First: Yellow mold (like green slime) is a classic trope of Dungeons and Dragons. From the very first OD&D books and modules, items that look 'golden' are often just colonies of yellow mold. There is a historical precedent for the description of yellow mold as a 'golden' color on objects.
Second: The players were in a hurry. They had 'cleared' the house. By that, they had (mostly) survived the major conflict in the area. As you can see from the transcript, they are moving forward in a hurried manner, attempting to 'clear' out the house.
Third: This is the subtle but key point to player agency. It is also often where people get upset about 'DM Fiat' and games being unfair.
The rule is, if it can have an effect or is important, then it must be mentioned to the players. In this case, on first observation the mold was mentioned.
"DM: Sure! You pull the door open. Inside it's empty, except for on a hanger, there is a dark cloak that looks like it has a gold lining."Compare this to "It is empty except for a cloak on a hanger".
Because they were in a hurry and I was feeling rushed, I did make the error of stating if asked if it was torn or decayed, "It looked fine". The original module I was cribbing from, does indeed note that the cloak is tattered. I am only human. However in true DM tradition, "It looks fine" doesn't mean that it is.
This style of agency is straight from the first edition DM guide.
DM: "The sacks hold rotten grain, so the cleric will go and help the magic-user as ordered. They find the refuse consists of castings, some husks of small victims of the spider, hide, bones, a small humanoid skull, and 19 silver pieces. Do you now fire the webs overhead?”
LC: ”Examine the skull first. What kind of humanoid was it? Can we tell?”
DM: ”Possibly a goblin. When you are looking at it more closely, you see that there is a small gem inside - a garnet.”
DM: "First, the others checking the containers find that they held nothing but water, or ore totally empty, and that the wood is rotten to boot. You see a few white, eyeless fish and various stone formations in a pool of water about 4' to 6' deep and about 10' long. That's all. Do you wish to leave the place now?"No search rolls. No character skills.
LC:"Yes, let's get out of here and go someplace where we can find something interesting."
OC: "Wait! If those fish are just blind cave types, ignore them, but what about the stone formations? Are any of them notable? If SO, I think we should check them out."
DM: "Okay. The fish are fish, but there is one group of minerals in the deepest part of the pool which appears to resemble a skeleton, but it simply - "
OC: "If the pole will reach, I'll use the end to prod the formation and see if it is actually a skeleton covered with mineral deposits from the water! I know the Shakespearean bit about a 'sea change'!"
Pure player skill.
Fictional Positioning: This discussion after the fact was delicate but necessary. One of the errors I made was that I called for saves before resolving the positioning of each character. Having rolled the die, forced Garth into a situation where he felt strongly that addressing his actual position would be regarding as cheating.
One offhand comment was made that this was a case of "Mother may I". This is explicitly incorrect, because he wasn't asking for permission. As I've noted, fictional positioning is addressed as a group with verisimilitude as the metric. Part of his hesitation was in wanting both to be fair and confronting what is traditional one of the vestiges of DM empowerment. Fictional Positioning in this game is something decided by the group. The players are in charge of their characters after all -- who have personalities of their own.