The symbol for candela is cd according to the NIST's website.
I gather you are mixing the concept of dimensions symbols (M, L, T) with dimensional analysis and energy units. The symbol for the mass dimension is M, it can be in any kg, mg, g etc, Length is L which can be millimeters, yards, kilometers etc. The dimension symbol for temperature is $\Theta$. So what should be the dimensional symbol for candela? It is J. Does it have to anything with Joule? No. It is by chance that the symbol matches with Joules. The problem is that English has only 26 letters. Have you heard of the pigeonhole principle?
In mathematics, the pigeonhole principle states that if $n$ items are put into $m$ containers, with $n>m$, then at least one container must contain more than one item. This theorem is exemplified in real life by truisms like "in any group of three gloves there must be at least two left gloves or at least two right gloves"
The same applies on symbols and quantities. From the official website of SI Units, this is the list of symbols to be used for these dimensions:
See the section on base units on Wikipedia.