Although the verb πτοηθῆτε is conjugated in the aorist tense, subjunctive mood, it is to be translated into English as an imperative: “Do not be terrified!” As Goodwin notes,1
In prohibitions, in the second and third persons, the present imperative or the aorist subjunctive is used with μή and its compounds.
This is the very matter in question. The verb conjugated as an aorist subjunctive can express a prohibition and be understood as though it were conjugated as a present imperative (both being preceded by μή and its compounds).
Thus, to answer your question: it is conjugated in the aorist tense, subjunctive mood. It should be translated into English as a negative imperative (prohibition).
1 Goodwin, p. 89, §259
Goodwin, William Watson. Syntax of the Moods and Tenses of the Greek Verb. Boston: Ginn, 1893.