I have a question about mutual masterbation risk.

No risk: of pregnancy. If using hand to genital contact only, there is negligible risk of STD transmission if skin is intact without open sores or bleeding and hands are washed well after masturbation before contacting ones own mouth, genitals or eyes. Unfortunately, we often touch our faces without thinking, so keep that in mind, but this can be a very satisfying way of pleasuring your partner.