$L = \left \{ <M> \right \}$ Where $M$ is some TM encoding.
Now, Let's analyze the given Two conditions for $M$ one by one.
1. $M$ is a $TM$ that Halts on All the inputs : From this condition/point, We have gotten to know that $M$ is a Halting TM, And since the language accepted by any Halting TM(HTM) is Recursive language, We can say that $L(M)$ is Recursive.
2. $L(M)$ = $L'$ for some Undecidable language $L'$ : Undecidable language means "Not recursive language". So, From this 2nd condition we have that $L(M)$ is some NOT Recursive language (i.e. Undecidable language)
So, Now, From the First condition we have "$L(M)$ is Recursive" and from the second condition we have "L(M) is NOT recursive"... So, Both are contradictory, Hence we can say that there is NO such $M$ possible. Hence, $L$ is Empty language. Hence, $L$ is Regular language and Hence $L$ is Recursive and decidable (Every recursive language is Decidable).