Q. What if someone says shalom aleichem to you while you are in the middle of the beracha (of Kidush Levana) can you respond?

A. (Please see question 575 in this forum) Shulchan Aruch (O.H. 66: 1) rules that in the middle of the brochos of Shema one is allowed to reply a sholom greeting to an individual that merits respect and honor such as an elder or a Talmid Chochom (Mishna Berura ibid.3). However, Mishna Berura (ibid. 2) also rules that since our custom is not to salute others during davening and people do not get offended if you don’t return their greetings since they understand that you are in the middle of a brocho, therefore it is prohibited to interrupt and return sholom. Avnei Yoshfeh (O.H. 50: 3) adds to the above that the expression of sholom in Kidush Levono is not a greeting that would require an answer at all. It is mainly to appease the one next to us for saying loudly “Tipol Akechem Eimassa Vephachad”, thus expressing that we are not addressing him.
However, as mentioned in the previous questions there are many reasons for the minhag of greeting Shalom Aleichem, still Horav Shlomo Miller’s Shlit”a opinion is that indeed one should not interrupt in the middle of the brocho.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a