What is on my crab's shell?

[mustangmamatn]

The guy that was maintaining our fish tank (doing water changes, supplying chemicals and fish/coral orders) flaked out on us. We have a 300 gallon reef and this little dude popped up with stuff on his shell over the weekend. Can anyone identify, please?

 

[Reefing Madness]

 

[eliramos]

better pic?

 

[nawilson89]

Bump!
OP with a better pic also please!