What fluid is this? (Inside wheel)

[70olbo]

I noticed this the other day:

I have since taken the wheel off and looked inside the drum at the wheel cylinder but saw no obvious fluid on the outside.

I looked while the wheel was off inside the wheel area and it looked like it could be coming from the area where the arrow is in the other picture. Does anyone have any ideas?

Is it safe to drive or should I wait until I have it looked at?

1970 k10 4x4