Wash the tail the best you can with dawn dishwashing liquid and warm water.
(Remember to rinse.) Snip the crap that won't wash off. No big deal. If you
can get another person to help with the washing, one person can hold him by
the back of his neck over a sink of soapy water while the other pulls his
tail down for a good scrubbing.

I have had similar problems with my cat - but closer to the anus.
Wiping carefully several times with wet toilet paper (wet with warm
water) removed it (if its not left for days to harden), bu the rest I
did carefully snip off with the fur that held it.  

Dont know why the cat isnt cleaning it like he used to .

Looks like he's suffering from diarrhoea that needs to be taken care of.

Carola

Sounds like you have a longhaired cat.  I have a similar problem with my cat
who is part persian.  Since the litter box is in the bathroom and her BMs are
usually strong enough that it will wake me out of a deep sleep, I usually just
go in while she's trying to cover it and close the door.  I keep a box of baby
wipes in there so I just have take one out of the box and wipe her bottom until
it's clean.  If I'm not home when she goes I check her to make sure that there
is nothing caught in her fir.  If there is we go into the bath, shut the door
and I put her in the sink where I can lift her tail and clip the dried up mess
out of her fir.  

Babie has never liked anyone doing anything around her bottom, but once the
bathroom door is closed she knows she won't be able to get out until she's
cleaned up and usually doesn't fight me as much so I can easily clean her up.

Kathy