Preparing to fill wall void to prevent damp Fillers, sealants and adhesives for all types of DIY work. Help, advice and information on all aspects of this subject

[Housediy]

I have removed the window trim on the inside because of a damp patch under the window sill. It has revealed what appears to be blocks of wobbly mortar or plaster on the inside with space around it. How can I seal it to prevent the damp? Do I need to remove the wobbly plater/Mortor or whatever it is first? If I do the hole will be bigger.
Thanks guys

[Housediy]

the large exposed bit at the bottom is wobbling. Is it plaster or mortar