Zooxanthellae live symbiotically within the coral polyp tissues and assist the coral in nutrition production through its photosynthetic activities. These activities provide the coral with fixed carbon compounds for energy, calcification etc. and oxygen The host coral in return provides the zooxanthellae with a protected environment to live within, and a steady supply of carbon dioxide for its photosynthetic activities.
Now, in a low light environment the zooxanthellae may not produce sufficient 'food' to meets the corals needs so it increases the supply of carbon dioxide resulting in an increase in the number of zooxanthellae, and thus an increase in the amount of 'food' to the coral. However, when the coral is exposed to a higher level of light the reverse can happen. The level of oxygen can reach levels dangerous to the coral and, to prevent damage to itself, it is able to ingest or eject zooxanthellae. Zooxanthellae are often golden or brown which is what you have seen ejected from the mouth of the coral.
High temperatures, (as little as 1 to 2 degrees celcius) can also result in a similar ejection of zooxanthellae.
These events are known as coral bleaching. If the event lasts a short time the remaining zooxanthellae are likely to multiply and return to normal levels, or the coral will injest new zooxanthellae from the water, with minimal long term effects. However, should the event be sustained and the corals loses all its zooxanthellae then it will eventually die.
Sven, in your situation the helio may have been somewhat stressed from the increase in light but then became further stressed from the temperature increase. Assuming that everything has now been stabilised I would expect the coral to recover.
I also don't know if it got stung by the nem ;), but I suspect the major problem was the temperature.
Hope this helps.