Xjyuk

I can't find this symbol used to indicate the disjoint sharp operator.

It's a # inside a circle. Is this symbol available in LaTex? if not, how can I draw it inside a mathematical equation?

You can use a scaled up version of bigcirc and ooalign:

documentclassarticle
usepackageamsmath,graphicx

makeatletter
newcommandmakecircled[2][mathord]#1mathpalettemake@circled#2
newcommandmake@circled[2]%
 begingroupm@th
 vphantombiggercirc#1%
 ooalign$#1biggercirc#1$crhidewidth$#1#2$hidewidthcr%
 endgroup

newcommandbiggercirc[1]%
 vcenterhboxscalebox1.4$m@th#1bigcirc$%

makeatother

newcommanddisjointsharpmakecircled[mathbin]#

begindocument

$Adisjointsharp B_Adisjointsharp B$

enddocument

I used mathbin but it could be mathrel depending on the meaning.

See https://tex.stackexchange.com/a/22375/4427 for a quick course on ooalign.

A have created your symbol with a combination of packages. Excuse me for the complicated code. Your symbol have the name disj. It is a variable name that you can changed. Here there is my genuine MWE proposal.

documentclassarticle
usepackageamsmath,amssymb
usepackageMnSymbol,scalerel
newcommanddisjmathrelbigcirclemkern-4muraise.3exllap$scaleobj.6#$
begindocument
$Adisj B=C$
enddocument

You can use TikZ to draw a circle node with a # inside. Using DeclareMathOperator from amsmath improves the spacing. The character should be a bit smaller than the current font, which you can do using smaller from the relsize package, to make sure it works in different fontsizes.

MWE:

documentclassarticle
usepackageamsmath
usepackagetikz
usepackagerelsize
DeclareMathOperatorchashtikznode[circle,draw,inner sep=0,outer sep=0]smaller[3]#
begindocument
$Achash B$

Huge$Achash B$
enddocument

Result:

Here is a possibility with stackengine and the bigovoid symbol from mathabx (without replacing the default maths fonts with the mathabx fonts):

documentclassarticle

usepackageamsmath
DeclareFontFamilyUmathxhyphencharfont45
DeclareFontShapeUmathxmn%
<-6> mathx5 
<6-7> mathx6 
<7-8> mathx7
<8-9> mathx8 
<9-10> mathx9
<10-12> mathx10 
<12-> mathx12


DeclareSymbolFontmathxUmathxmn
DeclareFontSubstitutionUmathxmn

DeclareMathSymbolbigovoidmathopmathx"EC

usepackagestackengine
newcommandOtagmathbinstackMathstackinsetcc#bigovoid

begindocument

[A Otag B = C ]

enddocument