You can use a variable.
If player is on icy plattform (overlapping for example) set value 1.
If value = 1, set deceleration to whatever you want.
Make an inverted event as well:
If value isn't 1 set deceleration to normal.
You could do it with just overlapping as well i guess, but it may be tricky to get it work properly that way.
Never really used plattform behaviour, so i'm a Bit unsure.