GSU why sps die with no obvious change or swings in parameters is extremely hard to nail down. If I had to guess I would say that something happened water chemistry or parameter wise while you were absent for a week. The tank looks young in the images and that may have contributed to it as well. Silicates usually are used by diatoms during the diatom bloom. Adding new equipment can introduce silicates but that usually does not initiate a bloom. Using some substrates not meant for reef tanks such as some types of sand will add a lot of silicate. Some have said that the use of egg crate contributes as well. If you are not seeing the beginning of purple coralline algae you might want to wait to introduce sps.