I was doing an inspection today and found this white substance on the concrete basement walls. The house is over 100 years old and the walls are made of rocks, but parged on the inside. The substance is white and powdery such as efforvescence, but in some areas there is this angel hair type stuff hanging on the wall. Hopefully the picture will show and I can get some ideas as to what it is. Do you think mold?
Efflouresence.
This might help you.
Interesting similarity of the problem.
http://www.aldonchem.com/popup-ab-efflorescence.htm
Marcel 
I believe you hit the nail right on the head. Thanks.
http://www.russellstone.com/csi/33.html
http://na.hansonbrick.com/en/techNotes/pdf/efflorescence.pdf
Efflorescence. The hairy looking stuff is extensive blooming.
I agree with the others that it is efflorescence. However, it almost looks as though mould may be tagging along for a ride?
That is not far from the truth, when you consider the magnitude of moisture intrusion of this extent to cause this efflorescence . Mold spores existing in most environments, will most likely find what is necessary to produce the real thing.
Since all they are missing is food, that comes by paint, glues, tile, and a multitude of sources.
So, by not seeing the whole picture here on this post, you could very well be correct in your assumption, and at this point, that is all it is on everyones comment. Without testing, one would never be sure. Comments of it might all be stipulation.
Good opinion of possibilities.
Thanks.
Marcel 