hello my cactus at one moment started to loose its green color and now looks as on the pictures. did i do something wrong? too much or too less water or light? please help
It appears to be a variegated monstrose form of Opuntia. It seems healthy enough for indoors, but I worry about the Sphagnum moss as it may indicate an improper potting mix. http://www.flickriver.com/photos/martenfisher/7452709950/ Here's a similar variety for comparison.
You could try more water. In my experience Opuntia like more water than most cacti, and I guess it is relativey hot were you live? Anyway I would check for root mealy bugs first, just in case. They are good at dehydrating even drought-tolerant species.
In my experience the sagging is most common on Opuntias that are too cold or too wet. They tend to shrivel in response to too dry conditions.
May be an Austrocylindropuntia subulata (formerly included in Opuntia). Best to replace the peat compost with a sandy / gritty mix.
