Cleaning mussels

Amy and Jacque are right, those are barnacles or limpets, nothing to worry about.

Here's SeriousEats' Kenji's longish article about mussels:

http://www.seriouseats.com/2014/11/how-to-clean-debeard-mussels.html

I buy farm-raised mussels and I don't do any prep work before cooking apart from a quick spray of water over the colander.

Those are likely tiny barnacles and no, you don't need to remove them. As long as they’ve been purged (for sand) and de-bearded they’re good to go. Just remember to discard any that didn’t open while steaming. Also, this is handy! https://food52.com/blog/6915-cleaning-and-cooking-shellfish

I can't tell from the photo's scale, but looks like they could just be small bits of barnacle - hard and raised slightly from the shells? I wouldn't worry about scraping them off. Just give the shells a good scrub to remove sand, dirt and if there are any "beards", pull those off just before cooking.