Definition of "hundreds of ice accretion"
I think that it's referring to hundredths of an inch, and "hundreds" is a typo for "hundredths". This FAA information sheet says:


  All ice accretion amounts will be reported to the nearest one-hundredth of an inch (0.01 in.). An example of a 1, 3, 6-hour ice accretion remark would be: “I1010 I3015 I6022.” This translates to 0.10 inches of ice in the last hour, 0.15 inches of ice in the last 3 hours, and 0.22 inches of ice in the last 6 hours.


This also matches the terminology used in some other weather reports, such as this one:


  Ice accretion was light, generally only a few hundredths of an inch...