וְכָל־בֶּ֣גֶד וְכָל־ע֔וֹר אֲשֶׁר־יִהְיֶ֥ה עָלָ֖יו שִׁכְבַת־זָ֑רַע וְכֻבַּ֥ס בַּמַּ֖יִם וְטָמֵ֥א עַד־הָעָֽרֶב׃ (פ)
Toute étoffe, toute peau, où il se trouverait de cette matière, sera nettoyée avec de l’eau et restera souillée jusqu’au soir.
Chizkuni
אשר יהיה עליו שכבת זרע, “(garments or skin) which have been splattered with semen of a human being;” even only partially; (Sifra) just as when this subject was discussed previously it referred to fresh, moist semen, so here too the Torah speaks of freshly emitted semen, not dried out semen.
Ibn Ezra
It would appear that both one’s skin and one’s clothes require immersion if semen is found on them, whether the semen be wet or dry. However, the truth is in accordance with the received tradition [Nidda 54b].