So on my ERB under "duty position" I have the title code SURPLUS SLDR. Does this mean I may be PCSing or am I just overstrength in my MOS?

You are currently not slotted into an actual paragraph and line number in your company. All admin stuff.

SPC (Join to see)
I did a quick Google search for your question and it could mean a couple of things:
You could be PCSing soon
Not in an authorized slot
S-1 hasn't inputted you into an authorized slot
And a few other reasons.
It's from a *.com site so take it for what it is, but the people who responded to it seem very knowledgeable about the topic.

http://community.armystudyguide.com/groupee/forums/a/tpc/f/ [login to see] /m/ [login to see]

What happens if you were a Surplus Soldier then you waited for the change then it changed to overstrength?