Hi Will,
That's your anti-halation layer coming out post-development as the fluo-pink dye.
Have you tested your fixer's capabilities to make sure it's not inadequate? Even if it hasn't, cold tap water rinse washing sometimes locks in the dye, even if you agitate wash vigorously. Wash at the same temperature range as development if you can. If you're using a fixer like Hypam [Ilford], it tends to be more prone than Rapid Fix [Ilford] and you might have to do what I do, and resort to a two-bath fixer method, preceding the fixer with a hyposulphite bath.
Depending on which negatives you're used to reading, Fuji Acros can seem strangely bland as a negative. It does have a grey base (most films do: some reversal ready films like Rollei RPX25 are clear).
RJ