Quoting Sean K. Reynolds on Spontaneous Casting:
Seriously, this is basic stuff. The language is a little weak in some places, but you know that there's a realistic way of interpreting it...and an unrealistic way of interpreting it. If there are two interpretations, and one seems too good to be true, go with the other one.
You can't really be confused by this. Because if you are confused by that, this cleric ability is going to blow your mind:
Spontaneous Casting: A good cleric (or a neutral cleric of a good deity) can channel stored spell energy into healing spells that she did not prepare ahead of time. The cleric can “lose” any prepared spell that is not an orison or domain spell in order to cast any cure spell of the same spell level or lower (a cure spell is any spell with “cure” in its name).
Heck, the above language doesn't even limit it to spells you have prepared! Why not use this ability to convert someone else's spell into a cure spell? If you know the enemy lich likes to use phantasmal killer, on your turn you should just spontaneously convert his spell into cure critical wounds! Enjoy 4d8+10 damage, lich!
Yes, his answer is blunt, but on point.
So, interpreting a single phrase out of the context of the ability, or even out of the context of the class, cannot be taken as rules as written simply because we are using english and it's interpretations as if it was part of the rule system, which it is not. The language is subject to flaws and interpretations, but when something is ambiguous or unclear, we have to look for a context, similar rules, general rules, or even FAQ's (and similar FAQ's) on the subject.
Now, on context, a cleric cannot spontaneously convert spells from another class because the other classes cannot cast cleric spells. Cleric spells are only given by the cleric class, and even oracles, that use the same spell list as clerics, have their own oracle spells. If we look up on the Oracle Class for reference of cleric spells, we will only find that they use the same spell list as the cleric.
A druid casts druid spells, thus those spell slots are invalid for spontaneous casting of the cleric class. Similarly, the spontatenous casting of the cleric class cannot use spell slots from the druid class. Classes that have abilities that can be used on multiple (different) class spells or allow to cast spells from another class, are usually explicitly called out as being able to to do (See Mystic Theurge and Arcanist).
The same issue will be seen on wizard/sorcerer abilities, and though they will often say "your spells" or "your spell slots", the rules as intended are that those spells are from the class you are currently reading.
This is a topic where the community extensively discuss this exact subject: Can a wizard 1/ cleric 1 spontaneously cast?