in islam, why is it haram for a strange guy to see a girl's period blood?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The question is quite unclear. Anyway, seeing the menstrual blood is not something prohibited and in principle a man is not prohibited to see the menstrual blood of a woman who is a non-Mahram (marriageable) to him. However, what is prohibited is talking to her about things that may lead to temptation, whether it is about menses or something else or her letting him see that (menstrual blood). All such things incite temptation and may lead them to Zina (unlawful sexual intercourse).

Besides, the relationship between a man and non-Mahram women is based on separating between them except in very narrow limits to avoid any mischief and keep away from falling in evil. For more benefit, please refer to Fatwa 81356.

Accordingly, it is not permissible for them to talk about such matters. Allaah The Almighty has prohibited the woman to speak softly to a man lest he should be moved with desire toward her; so Allaah commanded her to speak to him in an appropriate manner. In this regard, Allaah The Almighty Says (what means): {Then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech.}[Quran 33: 32]

Indeed, the one who learns that should know that it is not permissible for them (a man and a woman) to talk about such matters.

Allaah Knows best.