As JimD said, tap water is fine for cleaning your equipment. I generally use a 50/50 water/white vinegar solution to clean with.
About the back wall of your Biocube, it depends on your preference. Again, as JimD stated, some people like the artificial background, and some like the look of natural "stuff" such as coraline algae growing on it. If you prefer the artificial background, scrape off and remove whatever is growing on the back, if you like the coraline, just leave it be, and it will continue to grow.
Here is how I would clean the equipment you asked about:
Heater: I rarely clean my heater, when I do, I just rinse it off and clean whatever it has on it,
Skimmer: Obviously clean out the collection cup, I would rinse out the pump (maybe run it in very diluted vinegar from time to time) and just make sure it is free of debris,
Sponge Filter: I don't really use sponge filters (prefer live rock rubble), but I would defiantly rinse it out with water often, to keep it from becoming a nitrate factory,
Temperature Probe: Keep it clean of debris and anything else buy rinsing it in water.
Thats just how I would approach it, but keep in mind, I am not as experienced as most of the people on this forum, hopefully someone with more knowledge on the subject than me will help you out.
Hope this helps and good luck!
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