Is this mold?
Our bedroom smells musty, we've been sick and we found this gross stuff growing in our fireplace. Our estate agent says its just soot and dust, but the other symptoms make me think it's mold. I know we can get a proper test done but can you help visually? It's definitely got some dust and cobwebs, but feels like the underlying growth is more sinister.
It's difficult to tell from the pictures. I typically do not recommend a mold test; these tests often provide a false positive, simply because mold spores are very often floating in the air. That means you can have a positive result, but not have an actual mold problem.
If the bedroom smells musty, I'd do a visual inspection of the rest of the room. Check behind furniture, especially furniture placed against outside walls. If there is an attached bathroom, check in there as well. It is recommended that fireplaces be professionally cleaned every few years, so it might be worthwhile to have the fireplace cleaned and ask the cleaner what they think.
You may also want to check the humidity in your bedroom. You can purchase a simple relative humidity gauge at a local home improvement store. Indoors, you are looking to have a humidity range between 30-60%. That range makes it difficult for mold to grow. If your humidity is too high, you can look for simple fixes, such as running the bathroom fan during showers and for an additional 15-20 minutes afterwards, using a vent hood when you are cooking and periodically opening windows to release humid air (try 10 minutes of open windows once a week or so; it doesn't drop the temp of your house too dramatically).
If you do see mold elsewhere, I recommend mixing up a spray bottle of half water, half vinegar. You can spray this on the mold, and then scrub the mold away. This does two things - the water dampens the mold and presents mold spores escaping, and the vinegar kills off the mold.
I hope this helps!