Ну...

What does the following sentence means?

Ну, дык. Шарит, плесень...

I can't grasp it :(

"Ну" here means "well". "дык"= да/так (slang)
So it may be translated as "Well, yeah" or even "Sure!"
"Шарить" = to grasp (slang)
"плесень" here is unceremonious (friendly or rude, depending on context) form of address.
Well, if you grasp it, it could be said about you. :)

(Sorry about my English. May be somebody will explain better).

Yep, something like:

yeh, sure, that scum understands that.

That is a very special, and quite rude slang.