There is a very small risk in this situation.
When condoms are studied in a lab, the HIV virus cannot penetrate a latex condom. However, when condoms are studied in real world use, such as in couples in which only one person is infected, consistent and correct use of condoms is associated with about a 90% reduction in the risk of transmission of HIV. Many experts, quoting the studies in a lab, claim that the small failure rate in the real world experience reflects a failure or breakage of the condom. If the condom has partially slipped off, that would be considered a partial failure.
There is no ability of the studies to identify whether fluid from the vagina went into the condom, but the fact that it partially slipped off would be considered a failure regardless of whether it can be proven that vagina fluid entered the condom.
However, since the condom was still partly in place, there is some benefit. The average risk to the male of a single episode of unprotected penile-vaginal sex with an HIV positive female is about 0.04%, which also means that 99.96% will not get infected. If the condom provides 90% protection, then the risk decreases to 0.004%. If the lady is of unknown HIV status, your actual risk would be lower, by a factor that is commensurate with the prevalence of HIV among sex workers in your community. If she is HIV positive and is taking HIV medicines, that also would decrease the risk.
So, when you ask if you can catch HIV from this encounter, the answer is yes, but the risk is low. In the worst case scenario (that she is definitely HIV positive and not taking any HIV medicines), the risk would be about 0.004%.
If I can provide any additional information, please let me know.