MOLD is my guess also, especially since the lot grade sloped toward the house on 2 different sides and since the crawl space vents were at grade level in places.
"The floor joists have been coated with a substance we suspect is an anti-microbial compound, which is consistent with previous work by a mold remediation company. We suggest that you ask for full disclosure by the current owner as to the nature, purpose and scope of the work and obtain copies of any guarantees, permits, etc. related to this activity. Note that mold growth is symptomatic of high moisture conditions. Any remediation should have taken moisture control into consideration to prevent recurrence of fungi growth. More information about mold can be obtained on our website. (Note also that the sub-floor insulation in the crawl space appeared new, also consistent with mold remediation.)
The vapor barrier in the crawl space appeared new and it was unusual in that it lapped 2 feet up all perimeter walls and piers. We suggest that you ask for full disclosure by the current owner as to the nature, purpose and scope of the work related to this activity. Our experience is that vapor barriers may conceal but not correct a larger problem related to water intrusion in the crawl space."